# Cleaning solid surfaces with liquid interfaces and foams: From theory to applications

Cosima Stubenrauch, Wiebke Drenckhan-Andreatta

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Cosima Stubenrauch, Wiebke Drenckhan-Andreatta. Cleaning solid surfaces with liquid interfaces and foams: From theory to applications. Current Opinion in Colloid & Interface Science, 2024, 72, pp.101818. 10.1016/j.cocis.2024.101818. hal-05098491

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# Cleaning solid surfaces with liquid interfaces and foams: From theory to applications

Cosima Stubenrauch<fnref n="1" /> and Wiebke Drenckhan<fnref n="2" />

<figure><img src="image_2.png" /></figure>

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## Abstract

All personal and industrial cleaning sectors search for environmentally friendly methods to clean contaminated solid surfaces. Having relied for a long time on chemical and physico-chemical means with non-negligible environmental impact, these sectors are increasingly exploring the use of physical phenomena to improve cleaning efficiency. We summarise here recent progress in the area of cleaning methods that exploit the physical properties of liquid interfaces created by liquid menisci, bubbles, drops or foams. The high energy of these interfaces leads to a complex interplay between (1) interfacial forces, (2) viscous stresses created by flow fields under confinement, and (3) the capacity to adsorb solid and liquid contaminations. In appropriately designed cleaning processes, this interplay can reach an astounding efficiency, in many cases even with pure water, i.e. in the absence of any detergent. We will also show that whilst foams have always been assumed to be a mere side product of cleaning processes, recent research puts in evidence that they can actually be highly efficient cleaning agents, provided that their physical properties are properly chosen. We discuss a wide range of examples in which different interface-based cleaning methods have been investigated, including solid and liquid contaminations, or biological contaminations (bacteria, biofilms and biofouling).

## Addresses

Corresponding authors: Stubenrauch, Cosima (cosima.stubenrauch@ipc.uni-stuttgart.de); Drenckhan, Wiebke (wiebke.drenckhan@ics.cnrs.unistra.fr)

**Current Opinion in Colloid & Interface Science 2024, 72:101818**

This review comes from a themed issue on **Thin Liquid Films and Foams (2024)**

Edited by Sibani Lisa Biswal, Raymond R. Dagastine and Anne-Laure Fameau

For complete overview about the section, refer **Thin Liquid Films and Foams (2024)**

https://doi.org/10.1016/j.cocis.2024.101818

1359-0294/© 2024 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## Keywords

Cleaning, Foams, Bubbles, Drops, Interfaces, Triple-line forces.

## Introduction

The contamination of solid surfaces affects — to a greater or lesser extent — every area of life: medicine, food industry, household, means of transportation, electronic devices, or cultural assets, to mention just a few. The surfaces are contaminated as a result of exposure to e.g. microorganisms, soot, sand, pesticides, water, food, or chemicals depending on the field they are used in. The contamination can cause important damage to the original surfaces, can be dangerous for our health or it can simply be annoying. In most cases it has to be removed. Unfortunately, most current processes used to clean solid surfaces lack sustainability especially regarding the enormous use of clean, drinkable water and detergents.

**Water:** Let us illustrate the use of water with two well-known washing processes. (i) Car wash [1]: The average amount of water used in a commercial car wash is 150 L. With 250 million cars registered in the EU in 2020 this adds up to about $40 \times 10^9$ L. Globally the amount of water used for car washing annually is equivalent to the annual water needs of 33 million people. (ii) Semiconductor industry [2]: Water plays an essential role in cleaning the silicon wafers that are sliced and processed in computer chips. It reads in Ref. [2]: "Semiconductor manufacturing is a water and energy intensive industry. According to data announced in the sustainability reports of 24 semiconductor manufacturing corporations surveyed in this study, the total water and energy consumption were $750 \times 10^9$ L and $1.39 \times 10^{11}$ kWh, respectively, in 2021."

**Detergents:** Cleaning always involves the use of detergents, which have a serious impact on natural ecosystems [3]. The contamination of the environment by detergents affects fauna (e.g. animals drinking detergent-containing water) and flora (e.g. eutrophication). Furthermore, detergents entering wastewater treatment plants have a negative effect on the plants' performance. The *Federal Environment Agency* (Umwelt-bundesamt) of Germany published the following numbers in a report as of 2023 [4]: "In 2021 more than 1.4 million tonnes of cleaning agents and laundry detergents are sold to private end users in Germany, broken down as follows: ~540,000 tonnes of laundry detergent, ~231,000 tonnes of fabric softener,

<footnote n="1">¹ Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany</footnote>

<footnote n="2">² Institut Charles Sadron, University of Strasbourg, CNRS UPR22, 23 Rue du Loess, 67037 Strasbourg, France</footnote>
~664,000 tonnes of cleaning agents and care products, of which around 337,000 tonnes are dishwashing products. Moreover, unknown quantities of cleaning agents used in industrial applications add to this amount. From private households alone, ~440,000 tonnes of chemicals stemming from cleaning agents and detergents entered German wastewater systems in 2021.

In a nutshell, new, environmentally friendly cleaning methods that use less water and less detergents are urgently needed.

Gas-liquid interfaces and foams are commonly considered to have no cleaning effect. The latter are believed to be only a by-product of the cleaning process and an aesthetic add-on for the user [5,6]. However, current research shows that individual gas-liquid interfaces can actually have a pronounced cleaning effect (see e.g. Refs. [6,7] (Figure 1a–d)) and that foams can be used far more efficiently for cleaning solid surfaces than the

non-foamed detergent solution (see e.g. Refs. [6–9], Figure 1e–g). While studying the use of individual gas-liquid interfaces for cleaning is interesting from a fundamental scientific point of view, the use of foams for cleaning has a great application potential for several reasons. Firstly, since foams mainly consist of air the amount of detergent and water can be reduced by up to 90% compared to a cleaning with the corresponding non-foamed detergent solution. Secondly, foams can behave like a liquid or like a solid, depending on the stresses they are exposed to [10], and they “stick” naturally to a solid surface. This combination allows covering uneven, hard-to-access, and even vertical surfaces. Thirdly, foams are easy to create, apply and easy to remove. These three properties have been exploited in foam-based cleaning products for decades and will not be treated specifically in this review. However, recent research puts in evidence that interfaces, bubbles, and foams have much more to offer! As we will show in this review, different interface-driven mechanisms can make

<figure><img src="image_3.png" /><figcaption>**Figure 1**</figcaption></figure>

Examples of cleaning with interfaces: (a) Removal of bacteria from a solid surface by a bubble passing at low velocity (top row) and their displacement by the same bubble moving at higher velocity (bottom row) (adapted Fig. 4 from Ref. [16]). (b) Removal of human sebum from a silicon wafer at an air/water meniscus (clear central zone) (Fig. 4 from Ref. [6]). (c,d) Efficient reduction of biofouling by bubbling continuously along an inclined surface contained in the Atlantic ocean: (c) Close-up view of an epoxy panel after 5 weeks with bubbling on the top left (Fig. 5 from Ref. [17]) and (d) macroscopic view of PVC plates after 4 weeks with (bottom) and without (top) bubbling (Fig. 4 from Ref. [18].) (e) Removal of fluorescing oil film via imbibition into a foam seen from the bottom (adapted Fig. 3 from Ref. [8]). (f,g) Removal of contaminations from (f) historic art pieces (from Ref. [19]) and (g) a human arm (unpublished, courtesy T. Schad) by simply depositing an appropriately formulated foam on the surface for a few minutes.
the cleaning process much more efficient than that of
the bulk detergent solution. These effects include the
direct action of interfacial forces via triple lines, the
more indirect action of viscous stresses created by liquid
moving around interfaces (often in narrow wetting
films), or capillary stresses created by bubble assem-
blies. Interfaces can also carry contaminations away from
surfaces through interface-driven adsorption. We will
review in detail these mechanisms arising upon the
cleaning of solid surfaces in the presence of individual
inter interfaces, bubbles, drops or foams, together with the
accompanying literature exploring their use. While, in
principle, emulsions are also interesting for cleaning
purposes, no available literature explicitly reports their
use for the interfacial properties. This is why they are
not explicitly considered in this review.

For the sake of clarity and to avoid confusion, we will
define what we mean with *cleaning* and *cleaning efficiency* in
this review article. *Cleaning*: Solid or liquid (or both)
contaminations are removed from solid surfaces without
damaging it. Examples of contaminations which will be
dealt with here are particles (inorganic particles
(Figure 1g), bacteria (Figure 1a) or spores), solid coat-
ings (grease (Figure 1b,g), proteins, biofilms, biofouling
(Figure 1c and d)), and liquid oils (Figure 1e). *Cleaning
Efficiency*: We consider the cleaning process as efficient if
the removal rate is high, i.e. the more efficient the
cleaning process the more contaminations are removed
in a given period of time. What we will explore in this
review article is the role interfaces in general and foams
in particular play for cleaning contaminated
solid surfaces.

From now on we will speak about surfactants rather than
about detergents: the former are dealt with in scientific
studies, while the latter are used in real life applications.
A detergent consists of surfactants (main compound),
phosphates, fragrances, optical brighteners, and en-
zymes. For our discussion of mechanisms, however, it is
the surfactant that matters. We will discuss and explain
the mechanisms involved in cleaning (Section
Underlying mechanisms of interface-based cleaning)
before reviewing the literature about cleaning with
single gas-liquid interfaces (Section Cleaning solid
surfaces with individual interfaces) and foams (Section
Cleaning solid surfaces with foams), respectively. Con-
clusions and future perspectives will be discussed at the
end (Section Conclusions & future perspectives).

Even though interesting, for the sake of clarity, the
following four topics are excluded from this review: (1)
Foams are very often used to spread less liquid over a
surface, e.g. in car washing or carpet cleaning. In these
cases, the foam is simply the « carrier » of the cleaning
solution. The foam is supposed to decompose on the
surface to set the cleaning solution free and once this
has happened often a brushing of the surface or

mechanical action via rotating in a washing machine [11]
follows. The usage of foams in modern washing ma-
chines also decreases the penetration time of the sur-
factant into the fabrics because the foam offers a much
larger interface compared to the non-foamed detergent
solution [11]. *Not the foamed but the non-foamed cleaning
solution finally cleans the surface by solubilizing the contamina-
tions in micelles.* (2) There are studies which use foams
with a gelled continuous phase, so-called « gel foams »,
for cleaning solid surfaces. For example, Lascurain et al.
[12] study the cleaning of gypsum surfaces with a « gel
foam » whose continuous phase is gelled with agar, a
natural polysaccharide. Although the details of the
cleaning mechanism are not known yet, the « gel
foams » are used to control the amount of water released
on the surface. There is one exaption, though, which we
will adress in Section Conclusions & future
perspectives. *The mentioned « gel foams » but they are used
as « carrier » for water, which, in turn, is released and cleans the
surface by washing away the contaminations.* (3) Self-cleaning
of superhydrophobic solid surfaces is an important
research area (see e.g. Ref. [13]). The surfaces are
nanostructured such that water cannot spread out, i.e.
the water contact angle is 180°. In case there are solid
particles (microorganisms, soot, sand, dust ...) on the
surface, they are collected by the water droplets and
removed from the surface when the droplets run down
the surface. The same holds true for oil droplets on so-
called « omnophobic » surfaces [14]. *The gas-liquid in-
terfaces of the droplets (water or oil) do not clean but they are
used as « carrier » for the solid particles. It is the specific
structure of the solid surface that repels all liquids thus allowing
for easy droplet (and attached particles) removal.* (4) In ul-
trasonic cleaning, sound energy is converted into me-
chanical vibrations. The micro-bubbles in the liquid
vibrate under the action of the sound wave, thus
destroying and separating contaminations on the solid
surface of the object to be cleaned. In the literature we
reviewed, ultrasonic cleaning is mostly used to clean oil
contaminated surfaces (see e.g. Ref. [15]). However,
there is a negative side effect as regards sustainability
and recycling: very stable oil-in-water emulsions are
formed, i.e. the oil taken from the solid surface
now « contaminates » water. *Again the gas-liquid interfaces
are not directly involved in the cleaning process – contaminations
are simply removed by mechanical forces.*

Underlying mechanisms of interface-based
cleaning

The presence of interfaces leads to a variety of physical
mechanisms which can be exploited for efficient
cleaning methods. They can arise in the presence of a
single meniscus, a bubble or drop, or a foam. In princi-
ple, these mechanisms can also arise in emulsions. But
due to the absence of concrete examples in the litera-
ture, we will exclude them from the discussion to
simplify the reading.
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>

The four main mechanisms acting in the presence of interfaces and being useful for cleaning.

We describe here the four principal interface-driven mechanisms, which are sketched in Figure 2. As the reader will see in Sections Cleaning solid surfaces with individual interfaces and Cleaning solid surfaces with foams, efficient cleaning methods commonly combine several of these mechanisms.

The underlying concept of all mechanisms is the fact that an interface is associated with a characteristic interfacial energy *E*, which is proportional to its interfacial area *A* and its interfacial tension *γ*, i.e. *E* = *γ*A. In most cleaning contexts, the interfacial tensions *γ* (or interfacial stresses) are in the range of 1–100 × 10⁻³ N/m, creating associated interfacial forces which become quickly dominant with respect to other forces (gravity, adhesion, etc.), especially on small length scales.

In most cases, the main cleaning interface is that between a gas (often air) and the solvent used for cleaning. The gas/solvent interfacial tension is given by γgs. Even though the solvent can be any kind of liquid, in most cases it is water. In many cases, it contains additional cleaning agents, such as surfactants.

This allows us to give some numbers: For example, the water/air interfacial tension is 72 × 10⁻³ N/m, which

surfactants typically reduce to 25–45 × 10⁻³ N/m. Oil/water interfaces commonly have tensions of 20–30 × 10⁻³ N/m, which surfactants typically reduce to 1–6 × 10⁻³ N/m. For example, 1 mm of a liquid/liquid interface pulls with a force of approximately 1–10 µN on an object, while 1 mm of a gas/liquid interface reaches a force of approximately 10–100 µN. The same forces create a pressure jump across any curved interface (Young–Laplace law). For example, the pressure inside a 100 µm air bubble in water is ca 1500 Pa higher than the surrounding pressure.

### Triple-line forces

The first cleaning mechanism arises in the presence of “triple lines” (Figure 2. I), which are created between the solid surface, the gas, and the cleaning solvent. In equilibrium, these triple lines connect to the surface at the so-called “contact angle θ₀”, which can be estimated via the Young-Duprés law (Figure 1. I)

$$ \cos \theta_0 = \frac{\gamma_{g,sol} - \gamma_{s,sol}}{\gamma_{gs}}, \qquad (1) $$

where γg,sol is the interfacial tension between the gas and the solid surface, γs,sol between the solvent and the solid surface, and γgs between the gas and the solvent. This
equation is obtained by balancing the component of the interfacial stress of the gas-solvent interface which is tangential to the solid surface, i.e. $\cos\theta_0\gamma_{gs}$. The stress components normal to the solid surface lead to a triple-line force normal to the surface

$$F_{T,n} = \sin\theta_0 \gamma_{gs} L, \qquad (2)$$

where L is the length of the triple line along the surface. This force can pull efficiently on contaminations attached to the solid surface.

Moreover, most cleaning processes involve the sliding of such triple lines over the solid surface. In this case, it is known that depending on the type of surface (roughness etc.), the dynamic contact angle $\theta$ can be very different to the static contact angle $\theta_0$ given by Equ. (1), hence creating important tangential triple-line forces being able to slide and role contaminations across a surface. Triple-line forces arise also when the surface is covered by a liquid film and can help removing it.

## Viscous stresses

The second cleaning mechanism exploits viscous stresses which arise in particular in the presence of thin wetting films (Figure 2. II) and which are created when a bubble of radius R slides along a surface at a velocity U. The viscous stress $\sigma_V$ (defined as viscous force per surface area) exerted by the bubble on the solid surface depends on its sliding velocity U, on the viscosity $\eta$ of the liquid, and the thickness h of the wetting film. Since the shape of the bubble and h depend on U (h increases with

increasing U, see Figure 3a), the relationship between $\sigma_V$ and U is non-trivial and to first order captured by the proportionality

$$\sigma_V \sim \frac{\gamma_{gs}}{R} \left( \frac{\eta U}{\gamma_{gw}} \right)^{\alpha} = \frac{\gamma_{gs}}{R} C a^{\alpha}. \qquad (3)$$

$Ca$ is the “Capillary number”, which measures the relative importance of viscous and interfacial stresses. The proportionality factor of this relation depends on the configuration of the interface, while the value of the exponent $\alpha$ depends on the “mobility” of the gas/solvent interface [20,21], i.e. how easily the solvent can slide along the gas/solvent interface. If the interface is “immobile”, as is often the case in the presence of surfactants, the liquid experiences a no-slip condition at the gas/solvent interface and most of the dissipation occurs in the thin wetting film. In this case $\alpha \approx 2/3$. If the gas/solvent interface is “mobile”, however, the flow in the wetting film is of “plug-type”, as shown in the example of Figure 3a, and most of the dissipation occurs at the entrance and exit of the wetting film where the flow converges or diverges, respectively. Figure 3b shows that this leads to important, localised viscous stresses on the solid surface at the entrance and at the exit of the bubble. Since the stress changes sign, a passing bubble induces important stress fluctuations, which can be helpful for cleaning. For the total tangential viscous stresses exerted on the surface one finds the law of Equ. (3) with $\alpha \approx 1/2$.

<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>

(a) Pressure (colour code) & velocity field (black arrows) in the wetting film of a bubble sliding along a surface at increasing velocity U ($Ca = \eta U/\gamma_{gw}$). In these simulations the bubble is considered fixed, while the solid surface moves from the left to the right at velocity U. The solvent-air interface of the bubble is fully mobile. (b) Tangential viscous stress (force per surface area) acting on the wall for increasing sliding velocity, showing the dissipation field of the example $Ca = 4.73 \times 10^{-2}$ (Results are reproduced from Ref. [22].).
For a complete foam sliding at average velocity *U* along a surface, its liquid content (expressed by the liquid fraction *φ*) also plays a role. The less liquid a foam contains, the higher the friction forces it exerts on a surface. Quantitatively, this can be written as [20,23–25].

$$ \sigma_V = 3.8 \frac{\gamma_{gw}}{R} \phi^{-\frac{1}{4}} C a^2. \quad (4) $$

## Entry and spreading phenomena

The third mechanism involves “entry” and “spreading” phenomena (Figure 2. III) in the presence of a second liquid (often oil), considered as immiscible with the solvent used for cleaning. Since gas/solvent interfacial tensions can be fairly high, especially if the solvent is water, it can be energetically favourable to adsorb the immiscible liquid at the gas/solvent interface either by forming a liquid lens (III.a) or by spreading completely (III.b). In both cases, the new configuration needs to be energetically favourable, i.e. the total interfacial energy needs to decrease [26].

To form a liquid lens, the following condition for the “entry coefficient $E_C$” must be fulfilled

$$ E_C = \gamma_{gs} + \gamma_{os} - \gamma_{go} > 0. \quad (5) $$

To form a fully spread film the following condition for the “spreading coefficient $S_C$” must be fulfilled

$$ S_C = \gamma_{gs} - (\gamma_{os} + \gamma_{go}) > 0. \quad (6) $$

These conditions are either met by working with liquid combinations with particularly low interfacial tensions, and/or by choosing appropriate surface active agents. Once the second liquid has attached to or spread on the air/solvent interface of a bubble or a drop, it can be carried away by it.

## Imbibition

The fourth and last mechanism, “imbibition” (Figure 2. IV), is of similar spirit, but involves a more complex interplay of interfaces, requiring tightly packed bubble assemblies, i.e. foams. In such assemblies, bubbles deform away from being spherical, hence increasing their interfacial area *A* and therefore their interfacial energy *E* (see introduction of Section Underlying mechanisms of interface-based cleaning). Such an assembly has the capacity to “suck in” liquid via “imbibition” in order to reduce the energetically costly gas/ solvent interfacial area, i.e. to turn the bubbles back into more spherical shapes. This is trivial, if the liquid is the foaming solution itself. However, this mechanism can

also suck an immiscible liquid (often oil) into the foam, provided that the surface energy of the new liquid–liquid interface is lower than the energy gained by making the bubbles more spherical. In order to predict the imbibition capacity of a formulation, one can use geometrical approximations to define an “effective” interfacial tension $\gamma_{eff}$ of the foam containing the two immiscible liquids [27,28], namely

$$ \gamma_{eff} = \gamma_{gs} \left( 1 - 3 \frac{\gamma_{os}}{\gamma_{gs}} \right). \quad (7) $$

If this effective interfacial tension is positive, i.e. $\gamma_{eff} > 0$, then spontaneous imbibition of the immiscible liquid into the foam takes place. The speed of this imbibition depends on the bubble size and on the liquid fraction of the foam, i.e. on how strongly the bubbles are compacted against each other [27,28].

## Out-of-equilibrium effects

When considering the four mechanisms illustrated in Figure 2, it is important to keep two important aspects in mind:

(1) Until now we considered interfaces with a constant interfacial tension. However, in most real systems, **interfacial tension varies with time and along the interface**. This tends to arise in the presence of impurities or surfactants, which adsorb/desorb with time and with dynamic perturbations of the interface. Interfacial tension also depends on temperature, inducing local variations due to the evaporation of the solvent in cleaning processes. Hence, the values of the interfacial tension to be considered need to be carefully chosen, and, more importantly, one also needs to keep in mind that gradients of interfacial tension can create strong forces along interfaces (Marangoni effects), which can also contribute measurably to the cleaning process.

(2) **Wetting films play a very important role in nearly all the cited mechanisms.** For example, if a bubble is pressed against a solid surface, it forms a wetting film which drains and, if not protected sufficiently by stabilising agents or viscous stresses, it ruptures to create a triple line. In this case a mechanism dominated by viscous stresses (Figure 2.II) turns into one dominated by triple-line forces (Figure 2.I). A similar phenomenon arises when an immiscible liquid drop is pressed against a gas/solvent interface. The created solvent film has to drain and to rupture so that entry or spreading (Figure 2.III) of the drop can occur. This can also turn the imbibition mechanism (Figure 2.IV) into a spreading mechanism. And last but not least, triple lines moving quickly across a surface start to deposit
a wetting film beyond a critical velocity, changing
again completely the main mechanism of action.
Hence, when analysing these different configura-
tions, it is of utmost importance to keep in mind
that the characteristic time for drainage and rupture
of the wetting film (or of its creation) has to be
compared to the characteristic cleaning time. I. e.
even if certain configurations (and associated
cleaning mechanisms) are energetically favourable,
the system may not have the time to get there. This
is particularly the case in the presence of surface
active agents and also in dynamic systems, since
viscous forces tend to stabilise wetting films. We
discuss this in more detail in Section *Cleaning solid
surfaces with individual interfaces*.

Cleaning solid surfaces with individual interfaces

Investigations into the use of individual gas/solvent in-
terfaces for cleaning purposes have a reasonably long
history, being originally motivated by three main appli-
cations: (i) the removal of dust from wafer surfaces in
the semi-conductor industry, (ii) the self-cleaning of
solid surfaces by rain drops, and the transport of parti-
cles or micro-organisms through porous stone/soil due to
imbibition/drainage of water. In the meantime, the ef-
ficiency of individual interfaces for cleaning purposes
has been recognised as being useful in many other do-
mains, including the removal of bacteria, biofilms,
biofouling layers in marine environments or liquid con-
taminations. As sketched in Figure 4, such “individual
interfaces” are generated either by moving the solid to
be cleaned through the surface of a liquid (Figure 4a), or
by moving drops or bubbles along the solid surface

(Figure 4b and c). Typically, the efficiency is largely
increased by repeated dipping or by the passage of many
drops/bubbles. However, in contrast to the use of foams
for cleaning (see Section *Cleaning solid surfaces with
foams*), there are no collective effects between the
drops or bubbles. As already discussed in Section
*Underlying mechanisms of interface-based cleaning*,
even in the case of individual interfaces, a wide range of
different mechanisms can be exploited, depending on
the type of interface configuration, its dynamics and the
type of contamination to be removed. In the following
we summarise the existing literature, organised accord-
ing to the type of contamination to be removed. Un-
fortunately, only few of the authors we cite were aware
of the complexity of the underlying mechanisms and the
associated key parameters, whose analysis remains often
incomplete to draw reliable conclusions. Moreover,
many experimental investigations limit the analysis of
the cleaning efficiency to macroscopic before/after ob-
servations, making the assignment of the acting mech-
anisms a challenging task.

Removal of particles: from solid particles to bacteria
and spores

Many cleaning problems require the removal of particles
from a surface. These can be of very different nature,
including solid particles (dust) [7,29–41] or bacteria
[16,42,43]. They are characterised by their interaction
energies with the solvent, i.e. their wettability, which is
expressed by the contact angle θ₀,ρ which the gas/solvent
interface creates with the particle surface at the triple
line (see also Section Triple-line forces). The particles
attach to the solid surface by physical, chemical and/or
biological mechanisms creating a characteristic

<figure><img src="image_6.png" /><figcaption>Figure 4</figcaption></figure>

Examples of different interface configurations. (a) Meniscus created upon dipping the object to be cleaned at velocity *U* into a bath, with wetting film formation at high velocity. (b,c) Drop (top) or bubble (middle and bottom) sliding with velocity *U* along an inclined surface (b) or between two narrowly spaced surfaces (c). At low velocity *U*, triple lines are formed, while at high velocity *U*, the bubble slides on a wetting film (bottom row). The different velocities are indicated by different lengths of the arrows.
attachment energy which the cleaning mechanism has to overcome for particle removal. Interfaces come in handy in this case by (i) acting directly on the particles through *triple-line forces* (Section *Triple-line forces*) and/or by (ii) creating strongly localised shear zones and associated *viscous stresses* (Section *Viscous stresses*).

For *triple-line forces* to act, the triple lines have to be created on the surface of the particles (see also Section *Out-of-equilibrium effects*). This involves either the rupture of an air film (if a liquid front advances on a particle initially in air) or the rupture of the solvent film (if a gas front advances on a particle initially in a solvent). This means that the characteristic process time has to be longer than the characteristic life time of the wetting film. The latter depends on a variety of parameters: the wettability of the particle surface, the presence and properties of interfacially active agents, the velocity of the advancing front, or the viscosity of the solvent. If the wetting film on the particle has time to rupture, different configurations of the interface are possible, as shown in Figure 5. These configurations are controlled by the presence (or absence) of a wetting film on the solid surface, by the wettability of the solid surface (contact angle $\theta_{0,Sol}$) and by the wettability of the particle (contact angle $\theta_{0,P}$). The presence and thickness $h$ of the wetting film on the solid surface is controlled by the same parameters as discussed above for the wetting film on the particle.

Assuming perfectly spherical particles of radius $R_P$ and homogenous contact angles on the particle ($\theta_{0,P}$) and solid surface ($\theta_{0,Sol}$), one can estimate the maximum

triple-line force $F_{T,n}$ acting on the particle normal to the solid surface via [29,30]

$$F_{T,n} = 2\pi R_P \gamma_{gs} \sin^2\left(\frac{\theta_{0,P}}{2}\right) \cos \theta_{0,Sol} \quad (8)$$

in the absence of a wetting film (Figure 5b), or via

$$F_{T,n} = 2\pi R_P \gamma_{gs} \sin \psi \sin(\theta_{0,P} - \psi) \quad (9)$$

in the presence of a wetting film (Figure 5a,d). Here $\psi$ is the “opening angle” of the triple line on the particle as shown in Figure 5. Globally, it has been shown that a receding meniscus is much more efficient for cleaning than an advancing meniscus. The *tangential* components of the triple line forces can lead to sliding or rolling of the particles along the surface. The importance of these tangential contributions has been recognised only recently [41].

While the expressions (9)–(8) put in evidence the key parameters of the mechanism (particle size $R_P$ interfacial tension $\gamma_{gs}$, contact angles $\theta_0$, wetting film thickness $h$, and opening angle $\psi$) it is very important to keep in mind that in most cases they should only be considered as rough estimates. Particle shapes are often very different to spheres [39] and just like solid surfaces, they generally have inhomogeneous surface properties. More importantly, all considered processes are dynamic, hence dynamic contact angles tend to play a more important role than static contact angles. The difference in receding and advancing contact angles leads to additional symmetry breaking in the process and hence has been shown to impact non-negligibly the detachment

<figure><img src="image_7.png" /><figcaption>Figure 5</figcaption></figure>

Different interface configurations around a particle with (a,c,d) and without (b) wetting film of thickness $h$. Triple line forces act in configurations (a,b,d), while only viscous forces act in configuration (c).
mechanisms, in particular by increasing the importance of rolling the particles off the surface [41]. Due to the importance of the dynamic contact angle, one commonly observes an increasing cleaning efficiency with increasing interface velocity $U$. However, this is only true for a small range of velocities. With increasing interface velocity, a dynamic wetting film is formed on the solid surface (Figure 5d) whose thickness $h$ increases with increasing interface velocity $U$. If the wetting film is thinner than the size of the particles ($h < 2R_p$), the efficiency of the triple-line forces decreases with increasing wetting film thickness $h$ (since the opening angle $\psi$ decreases in Equation (9)). Once $h$ is larger than the particle (Figure 5c), and/or the wetting film on the particle does not have the time to rupture, the triple-line forces can no longer act. Viscous stresses on the particle create then the dominant forces. The total viscous force $F_V$ created by the shear flow on the particle can be estimated as

$$F_V \sim \sigma_V R_p^2. \quad (10)$$

The magnitude of the viscous stress $\sigma_V$ depends on where the viscous dissipation arises (in the wetting film or in the meniscus), as discussed in Section Viscous stresses (Equation (3)). The viscous forces — and hence the associated cleaning efficiency — increase with increasing interface velocity. However, globally, viscous stresses tend to be less efficient than triple-line forces. While triple-line forces act strongly vertically to the surface, viscous forces act mostly tangentially to the solid surface, hence contributing most easily to a displacement of the particles along the surface (see bottom of Figure 1a), rather than its complete removal (see top of Figure 1a). Recent work [9,44,45] (see also Sections Removal of coatings: from lipids to biofilms and Removal of particles: from solid particles to bacteria and spores) has shown, however, that strong fluctuations in the viscous stresses created in the meniscus or by non-homogenous interface movements (for example by bouncing bubbles) can increase the efficiency of viscous detachment.

In summary, the general picture of interface-based removal of particles from solid surfaces given by the current literature is as follows: the most efficient cleaning is done by triple-line forces arising in a receding meniscus. For maximum efficiency, triple lines have to be formed on the particles and on the solid surface, requiring slow motion of the meniscus to ensure the rupture/absence of wetting films. With increasing interface velocity, dynamic contact angles on the particle introduce additional tangential forces, leading to rolling and sliding of the particle on the surface, thus increasing the detachment efficiency. Once the triple lines are formed, the particle often actually remains

attached to the interface and can be carried away with it. With further increasing interface velocities, wetting films are created on the solid surface and/or on the particle surfaces, replacing progressively the triple-line forces by viscous forces. Even though the efficiency of the viscous forces increases with increasing interface velocity and especially with velocity fluctuations, they commonly remain less efficient than triple-line forces.

## Removal of coatings: from lipids to biofilms

Here we consider the removal of coatings from a solid surface, including solid grease [6], inks and polymer coating [46], biofilms [47–51], or layers of biofouling [17,18,52,53] (Figure 1c and d). This coating can be more or less “soft” with the condition that it does not flow. The fundamental mechanisms are the same as discussed in Section Removal of particles: from solid particles to bacteria and spores with the key difference, that the arising forces need to act on a continuous surface, rather than on a particle. In contrast to the removal of particles by interfaces, scientific investigations into the removal of coatings are much less developed with rather scattered literature lacking a unifying picture. We shall provide here a brief summary of what has been done in this area up to now.

One of the most efficient mechanisms is again a triple line pulling directly on the coating (Section Triple-line forces). For this triple line to be present, the meniscus needs to advance sufficiently slowly to avoid the formation of a wetting film or allow for its rupture [46]. The efficiency of the triple line is controlled by the wetting properties of the surface and also by its roughness. Figure 1b shows the example of lipids (human sebum) efficiently removed from the surface of a silicon wafer by the meniscus which is created when the wafer is dipped into a water bath [6]. Jones et al. [6] showed that the dipping velocity was not very important in the investigated ranges, but that the presence of the interface and of the surfactant in the solvent was a key element. Khodaparast et al. [46] showed that at a slow dipping velocity, hydrophobic coating (inks and polymer coatings) can be very efficiently removed by the triple line forces acting during the slow passage of a pure air/water interface, profiting also from the fact that the coating remained attached to the interface. Increasing viscous stresses upon increasing dipping velocity dramatically reduced the removal efficiency.

Upon increasing the interface velocity, wetting films are deposited on the surface and the triple-line forces are replaced by viscous stresses as discussed in Section Viscous stresses. Most investigations in the literature have been undertaken in the context of biofilm removal (for example for the cleaning of bioreactors) or for biofouling treatments in the ocean, and involve the sliding of bubbles across a surface contained in a solvent to exert the viscous stresses. It has been shown that
viscous stresses have to be above a critical value depending on the type of surface considered. Without surprise, maximum viscous stresses are shown to be the most efficient, requiring thin wetting films at high bubble speeds. These are achieved by ensuring that bubbles are efficiently pressed against the surface (for example through gravity or confinement) whilst moving along it. This leads to an optimum tilt angle of the surface to be cleaned if exposed to rising bubble streams, which Hooshanginejad *et al.* [51] find to be close to 20° for their conditions. In the case of biofilm applications, it has been shown that above a characteristic viscous stress, the growth of the biofilm is actually enhanced, hence inversing cleaning efficiency. In this case it has been shown that it is important to apply continuous bubble streams from the very beginning to avoid the formation of biofilm/biofouling layers.

Last but not least, detailed measurements of surface stresses in the presence of bubble streams have put in evidence the importance of stress fluctuations for efficient coating removal [44,45]. The precise mechanism has not been clarified yet, but we suspect that the fact that stress fluctuations and associated deformation fields create normal forces to the coating surface which helps with the removal of the coating.

In summary, while viscous forces exerted by interfaces have been exploited to a certain degree in the removal of solid coatings, the efficiency of triple-line forces remains

largely underestimated at this stage and should be considered more thoroughly for future investigations.

## Removal of liquid oils or grease

Here we consider the removal of liquid contaminations (often oil) from a solid surface by an immiscible solvent (the case of a miscible solvent being trivial). Different mechanisms can act here, as already discussed in Section *Underlying mechanisms of interface-based cleaning*. A triple line can be formed between the gas, the solvent and the oil, pulling efficiently on the oil film and leading to its progressive detachment. The oil can attach to the air/solvent interface as a lens or spread fully across it (Figure 2.III), allowing its progressive removal. Even though the fundamental mechanisms of this latter process have been investigated in detail in other contexts (in particular in the context of antifoams [26]), its systematic exploitation for cleaning purposes (in the absence of ultrasound) has been reported only in two publications [15,54], as far as we are aware. There, the authors investigate the cleaning of metal surfaces covered by liquid oil using microbubbles in different solvents. They find that the cleaning in the presence of microbubbles is much more efficient than the cleaning with SDS solution only, providing the following interpretation (sketched in Figure 6): the microbubbles enrobe themselves with the oil and coalesce at the solid surface, creating increasingly large bubbles. Once the coalesced bubbles are large enough, buoyancy allows them to detach from

<figure><img src="image_8.png" /><figcaption>Figure 6</figcaption></figure>

(A) Adapted sketch of liquid oil removal by microbubbles (from Ref. [15]). (B) Oil imbibition into a 3D-foam (reproduced from Ref. [55]). (C) Investigation of oil imbibition into a 2D-foam of decreasing liquid fraction from left to right (reproduced from Ref. [56]).
the surface and to rise to the top of the solution. Here
they coalesce with the surrounding air creating an oil
film at the top of the solution which can be creamed
off. The authors put in evidence some tendencies, for
example, an increasing cleaning efficiency with
decreasing oil viscosity, or the importance of working
with microbubbles and (some) surfactant.

Liquid oil films can also be removed by viscous shear forces combined with interfacial instabilities. However, we are not aware of investigations in this area.

In summary, it seems to us that the area of liquid oil/
grease removal by individual interfaces remains largely
unexplored and deserves much more attention and sys-
tematic investigation in the future. Solid oil (Sections
Removal of coatings: from lipids to biofilms and Removal
of particles: from solid particles to bacteria and spores)
can, for example, be liquified via temperature increase or
addition of liquefying components, to be removed as a
liquid. For the removal of a continuous oil film from a
solid surface one needs to keep in mind that not only the
question of the attachment of the oil at the gas/solvent
interface is important, but also the detachment of the
bubble carrying the oil from the surface. For the latter, an
oil filament drawn within the solvent has to rupture.

Cleaning solid surfaces with foams

While in Section *Cleaning solid surfaces with individual interfaces* we have treated the cleaning effect of individual interfaces, we will now turn to the questions of how to clean solid surfaces with many interfaces, i.e. with foams. Note that high internal phase emulsions should also be of interest for cleaning. However, we did not find any relevant literature on the subject, which is why we concentrate purely on foam-based cleaning.

There are two fundamentally different approaches,
which we will call *static foam cleaning* and *foam flow cleaning*,
respectively. Both techniques are used to clean solid
surfaces contaminated with particles, coatings, or liquid
oils. We will first define *static foam cleaning* and *foam flow
cleaning* before giving examples of the cleaning of solid
surfaces with either of the two techniques in Sections
*Removal of particles: from solid particles to bacteria and
spores*, *Removal of coatings: from lipids to biofilms and
Removal of liquid oil and grease*.

*Static foam cleaning:* In this case, the foam is simply brought into contact with the surface to be cleaned and allowed to sit there for a given time before being removed [6,8,12,19,27,28,57,58]. The mechanisms which clean the surface are *triple-line forces*, *viscous stresses*, and/or *imbibition* (see Section *Underlying mechanisms of interface-based cleaning*). *Triple-line forces* occur when the wetting film of the bubbles ruptures so that the gas gets in direct contact with the solid surface (as shown in

Figure 7a). If the wetting films remain intact, viscous forces arise upon bubble motion across the surface. Although the foam itself does not move with respect to the surface, rapid bubble rearrangements happen in the foam due to foam ageing effects (coarsening, coalescence, drainage) [21,59,60]. These dynamic effects cause local wall shear stresses, which, in turn, have a wiping effect as explained in Refs. [8,57]. *Imbibition* occurs when the foam is brought in contact with a liquid deposited on a solid surface. The liquid is sucked into the foam to reduce the interfacial area between water and air, provided the interfacial conditions are fulfilled as explained in Section *Imbibition*. The amount of liquid and the rate at which it is sucked in, depends on the liquid fraction of the foam and on its bubble size. The smaller both are, the larger the volume of liquid which can be sucked in – but the slower the process [27,28,55]. Internal foam dynamics can also help here to accelerate the process [61].

*Foam flow cleaning:* Here the foam is moved along the surface with a fixed velocity ([7,9,62–66]) and it is the action of wall shear stresses that clean the surface. During *foam flow cleaning* a thin wetting film is formed on the surface to be cleaned. The movement of the foam induces fluctuations of the film thickness and of the wall shear stresses. The higher the velocity of the foam, the thicker is the wetting film and the higher is the wall shear stress (see Equ (3) in Section *Underlying mechanisms of interface-based cleaning*). Once the contamination is detached from the surface it is either located in the continuous solution of the foam or attached to the bubbles and it is removed with the removal of the foam.

Removal of particles: from solid particles to bacteria
and spores

*Static foam cleaning:* As defined above, in *static foam cleaning* the foam is simply brought into contact with the surface to be cleaned. The goal of Dari et al. [66] was to remove bacteria spores from solid surfaces with *static foam cleaning* after having been successful with *foam flow cleaning* [9,64,66] as is outlined further below. They found that cleaning of spore-loaden surfaces with static foams generated with the double-syringe technique is far more efficient than cleaning these surfaces with (i) water, (ii) non-foamed surfactant solutions, or (iii) foams generated via handshaking. The authors explain the observed cleaning efficiency in the presence of foam by *imbibition*. However, the explanation is not in line with the definition of *imbibition*, which is restricted to the absorption of liquids by a foam with low liquid content with a clearly defined driving force as explained in Section *Imbibition*. The cleaning mechanism is thus unclear. A possible explanation could be that the surfaces were not “dry”. The surfaces were treated with a pore suspension and then dried for 1 h at 30 °C. This heat
<figure><img src="image_9.png" /><figcaption>Figure 7</figcaption></figure>

(a) Illustration of the tribological effect associated with a non-wetting and a wetting situation, respectively, at the solid surface. (b) Image of a wafer after a static foam cleaning at ×5 magnification, showing distinct patterns in the sebum. Figures (a,b) reproduced from Ref. [6] with permission from Elsevier © 2016. (c) Schematic drawing of the fluctuating wall shear stress induced by bubbles of different size that move along the surface. According to Dallagi et al. [9] these fluctuations are the reason why the flowing foam cleans better than the flowing non-foamed surfactant solution where no fluctuations of the wall shear stress occur. Figure reproduced from Ref. [9] with permission from Elsevier © 2022.

treatment may not be enough to dry the surface. If this was the case, it is not the spores that were sucked in but the suspension, i.e. the aqueous solution containing spores. This explanation would be in line with the observations made by Schad et al. [57] who cleaned surfaces that were contaminated with oil and soot. Here it was the oil that was sucked in by the foam and the oil, in turn, served as vehicle for the soot particles. Again, we are talking about the imbibition of a liquid (containing a solid) and not about the imbibition of a solid for which there is no driving force in the first place. However, Dari et al. [58] may also want to consider that triple-line forces could act in their system as discussed in Section Removal of particles: from solid particles to bacteria and spores.

**Foam flow cleaning:** In three studies of Dallagi et al. [9,64,66] *foam flow cleaning* is used for the removal of bacteria spores from solid surfaces. Their focus is on improving surface hygiene in the food industry. As model contamination, the authors use hydrophilic and hydrophobic Bacillus spores with sizes of ~1 µm. As cleaning

foam, a very wet foam with a liquid fraction of 50% is used. They found a more efficient cleaning with the flowing foam compared to the cleaning with the non-foamed surfactant solution. Note that in the latter case, the non-foamed surfactant solution flows also along the surface. Dallagi et al. [9] controlled the flow conditions such that the average, flow-induced mechanical stresses at the surface to be cleaned is comparable with and without foam. The better cleaning results obtained with the flowing foam are explained by the authors as being due to local fluctuations of the wall shear stresses, which are caused by bubbles with different sizes that move along the surface (s. Fig. 4 in Ref. [9] and Figure 7c). As already mentioned above, the authors incorrectly use the expression *imbibition*. The observation that the foam takes on board the spores is probably not due to imbibition but to absorption (as correctly described by the authors on page 11 in Ref. [9]).

Andreev et al. [7] also used *foam flow cleaning* but with a different set-up. Here the surface to be cleaned is vertically immersed into and withdrawn from an immersion
cell as opposed to the horizontal surface used by Dallagi et al. [9]. The immersion cell contains either an aqueous surfactant solution or a foam and the contamination to be removed is 90 nm Si3N4 particles. Note that the authors use the expression “wall shear rate” instead of “wall shear stress” but the underlying mechanism of viscous stresses is the same as the ones described by Dallagi et al. [9]. Andreev et al. [7] also observed that the foam cleans better than the non-foamed surfactant solution. Moreover, they find that the lower the liquid fraction of the foam the better the cleaning efficiency. The explanation given is comparable to the one of Dallagi et al. [9]: the flowing foam bubbles increase the “wall shear rate” and hence the wall shear stress compared to a non-foamed solution. What is different to Dallagi et al. [9] is the observation that the more bubbles (the lower the liquid fraction of the foam) the better the cleaning efficiency. Dallagi et al. [9] observed that with decreasing liquid fraction (they tested 50%, 40% and 30%) the cleaning efficiency decreases. An explanation for this observation is not given. We believe that some of these apparent contradictions arise from the fact that the used foams are often inhomogeneous in terms of liquid fraction. For example, a foam with 50% liquid fraction separates rapidly into a foam floating on bulk liquid. Future work therefore needs to establish carefully the foam conditions.

## Removal of coatings: from lipids to biofilms

**Static Foam Cleaning:** As discussed in Sections Underlying mechanisms of interface-based cleaning and Cleaning solid surfaces with individual interfaces, Jones et al. [6] studied the influence of a liquid meniscus — *i.e.* of a triple line moving along a surface — on the cleaning of solid surfaces coated with a film of human sebum (= oily substances plus lipids). They also carried out some preliminary cleaning tests with foams where they simply put the surface in contact with foam. What they found is a foam pattern on the surface (see Figure 7) which suggests the removal of sebum in some parts. They conclude that this can only happen if the surfactant solution does not wet the wafer, a deeper discussion, however, is lacking.

Human sebum (amongst other contaminations such as dust, soot, pesticides etc) is also a contamination on historical surfaces after having been touched by generations of people. Schad et al. [19] successfully cleaned such contaminated historical surfaces (Figure 1f) and ascribed the cleaning — incorrectly (see Section Removal of particles: from solid particles to bacteria and spores) — to imbibition and — correctly — to viscous wiping effects. Similar effects have been observed on human skin (Figure 1g). A complete explanation of the observed cleaning efficiency remains to be established.

**Foam flow cleaning:** Dallagi et al. [65] use foam flow cleaning not only for the removal of Bacillus spores (see Section Removal of particles: from solid particles to bacteria and

spores) but also for the removal of biofilms — they tested the technique on three different biofilms. As was the case for the spores, they found an improved cleaning with the flowing foam compared to the cleaning with the flowing non-foamed surfactant solution. The explanation given by the authors is the same as the one given for the removal of the spores, namely local fluctuations of the wall shear stresses induced by bubbles of different size that move along the surface. Again, the fact that the foam removes the biofilms cannot be due to imbibition as argued by the authors.

## Removal of liquid oil and grease

As regards the removal of oil from solid surfaces, we only found references for static foam cleaning which is why we do not distinguish between static foam cleaning and foam flow cleaning but between fundamental and applied studies.

**Fundamental studies:** The focus of the fundamental studies is on the processes that occur when an aqueous foam gets into contact with oil. Studies on oil being sucked into a single Plateau border [67] and into a simple foam film network [61] were followed by the pioneering work of Sonoda et al. [68]. In this study the imbibition of oil into a two-dimensional foam was studied. The authors either described the mechanisms with scaling laws [61,67] or explained the phenomenon with competing interfacial tensions [68], as discussed in Section Underlying mechanisms of interface-based cleaning. Note that in this early work the word imbibition was not used yet, while the same authors talk about imbibition in a follow-up study [56] (Figure 6C). The word imbibition in relation to oil being sucked up by a foam was taken up again (after having been used by Schramm et al. [69] in 1990 in connection to oils acting as defoamers) by Mensire et al. [27] shortly after the work of Sonoda et al. [64] was published. Mensire et al. [24,25] studied the imbibition of oil into foams with very low liquid fractions of 0.07–0.7 vol% (Figure 6B). Other than Sonoda et al. [68], Mensire et al. [28] considered the importance of the air–water interfacial tension and correctly ignored the air-oil interfacial tension. The air-oil interfacial tension plays no role for imbibition since the moment the oil is covered by the foam a water–oil interface is created. Mensire et al. [28] also considered the bubble radius and thus the capillary forces. The authors conclude that two criteria define the imbibition efficiency. (i) The first one is the ratio between the oil-water and the air–water interfacial tension which controls the imbibition strength (see also Section Imbibition). (ii) The second one is the Bond number Bo = $ρg/γR^2$, which compares the importance of gravity and interfacial stresses for bubbles of radius R. The lower Bo, the less the effect of gravity and the more liquid is imbibed. Decreasing the bubble radius is therefore the most efficient option for reducing Bo. A review article entitled “Stable oil-laden foams: Formation and evolution” was published by
Mensire and Lorenceau in 2017 [55]. The last fundamental study was carried out in 2018 by Kusaka et al. [56] in which the role of the liquid fraction was investigated. As opposed to the foams with very small liquid fractions used by Mensire et al. [27,28], the imbibition capacity of foams with high liquid fractions up to 25 vol % was studied. It was found that imbibition occurs up to a liquid fraction of 15 vol% (Figure 6C), while at higher liquid fractions no oil is sucked in by the foam. Note that Kusaka et al. [56] did not determine the average bubble size – it can only be estimated from Figure 3 in Ref. [56]. We are emphasizing this point because it is the combination of low liquid fraction and small bubble size that favours imbibition. In other words, if one increases the liquid fraction (decreases the imbibition degree), a decrease of the bubble size (increase of imbibition degree) counteracts this process. Thus, foams with liquid fractions that are even higher than 15 vol% may be also used for oil take-up if the bubble size is decreased accordingly. The authors of the mentioned studies were well aware of the cleaning potential of oil imbibition, but they did not explore this point.

*Applied studies:* The phenomenon of oil imbibition was used by Schad et al. [8,57] to clean oil-contaminated surfaces. In these studies, a foam was brought into contact with an oil-contaminated surface and left there for a specific period of time. The foam was prepared with the double syringe technique [70] in order to obtain foams with low liquid fractions (liquid fraction ≤5%) and small bubbles (diameters <50 µm), i.e. conditions under which imbibition is expected to be very efficient as regards the amount of liquid that is sucked in by the foam. Comparing the cleaning efficiency of stable (the gas phase (N₂) contained traces of perfluorohexane) with unstable foams (no perfluorohexane in the gas phase), the authors found that the latter clean much more efficiently (see Figure 1e). This comparison revealed that oil imbibition is not the only mechanism involved in the cleaning of oil-contaminated surfaces since imbibition occurs in both the stable and the unstable foam. As already mentioned, rapid bubble rearrangements happen in the foam due to foam ageing effects (coarsening, coalescence, drainage) and these dynamic effects cause local wall shear stresses, which, in turn, have a wiping effect. They also accelerate imbibition [61]. These rapid rearrangements do not happen in the stable foam in which we neither have coarsening nor coalescence during the contact time with the surface (5 min). Note that the effect of perfluorohexane on coalescence was shown only recently by Steck et al. [71,72]. The combination of imbibition and viscous forces (wiping) is the reason for *static foam cleaning* of oil-contaminated surfaces. The role of triple-line forces or oil spreading in the air/solvent interface is probably negligible in this context, since it would lead to an antifoam-effect of the oil [26].

Conclusions & future perspectives

We provided an overview of the state of the art of interface-based cleaning methods exploiting physical mechanisms which arise in the presence of liquid interfaces created by menisci, bubbles, drops and foams. We showed that the same kind of interface geometry can create very different mechanisms, ranging from triple-line forces and capillary stresses, via viscous stresses, to the adsorption of contaminations at the interface. Depending on the formulation and process conditions (presence of surfactants, liquid viscosity, advancing speed, etc.) either one of the mechanisms dominates – or several mechanisms act in synergy, sometimes providing impressive cleaning results. The literature shows that in some cases, nearly perfect cleaning can be achieved with water only (i.e. in the absence of any surfactants) if the physical mechanisms are appropriately exploited.

The existing literature also puts in evidence the importance of dynamic effects. For example, wetting films play a key role in nearly all mechanisms. Their rupture or creation changes dramatically the underlying mechanism(s). The same holds for dynamic contact angles, which can be very different from the static ones, leading to important changes in the force balances. Last, but not least, temporal or spatial variations of interfacial tension arise easily under dynamic conditions, creating additional stresses with non-negligible impact. Similarly, important interfacial tension gradients arise also in the presence of temperature gradients. One example are evaporation-induced temperature gradients, whose importance has recently been highlighted for foam films and foams [73]. Such effects surely deserve more attention in future research.

In order to interpret interface-based cleaning methods, it is therefore of vital importance to avail of a clear physical understanding of the system to identify the key control parameters and to characterise the ongoing processes at micro- and macro-scale. Much of the available literature suffers from the fact that (i) not all the key parameters are reported, (ii) vocabulary is not always clearly defined, and (iii) the experimental investigations are often restricted to the analysis of the surface “before” and “after” the cleaning process. This has led to quite some confusion in the past, for example concerning the sometimes contradicting role of the sliding velocity of the interface across a surface. These contradictions are easily resolved once the different velocity-dependent regimes are identified. In the case of particle removal from a surface, a coherent picture of the different static and dynamic mechanisms has been established over the last decade. The same kind of approach is starting in other areas (biofouling, liquid oil removal, etc.), yet it seems to us that closer collaboration of biologists and physical chemists with physicists is required to elucidate the underlying
mechanisms. In particular, we encourage researchers to undertake maximal efforts to obtain a microscopic analysis of the interface configuration in their process (for example through (confocal) microscopy, interference optics, etc.) in order to identify clearly the acting cleaning processes.

Some of the reported methods are already well understood and advanced (for example particle removal), while others are still in their infancy. For example, foams have, for a long time, been completely underestimated (or better ignored) for explicit cleaning effects, since they were mostly considered as side products, or for their capacity to spread and stick cleaning solutions onto large surfaces. Recent work shows that the cleaning capacity of foams can be impressive. However, more systematic work needs to be done to rationalise the underlying processes and thus to develop optimised and flexible foam-based cleaning methods. Surprisingly, no investigations have been found in which high internal phase emulsions are used for cleaning, which should surely be an interesting research direction to follow up!

In some areas beautiful academic work exists on interface-driven mechanisms, but without being explicitly exploited for cleaning purposes. This is, for example, the case for (i) liquid oil uptake by bubbles or foams [20,55] and (ii) aqueous solution uptake by « gel foams » [74]. In the latter case, the authors envision the cleaning of nuclear reactors with « gel foams ».

In summary, we think that the subject of interface-based cleaning methods has a bright future ahead of it, asking for close collaboration between physics, physical chemistry, biology and engineering. We are convinced that an optimised exploitation of the synergy of the different physical mechanisms will allow to reduce significantly the large quantities of detergents and water which are currently used in most cleaning methods. The final goal must be to pave the way to environmentally friendly and sustainable cleaning processes.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

No data was used for the research described in the article.

## Acknowledgements

Tamara Schad is thanked for discussions and proof-reading. Arnaud Saint-Jalmes, Anne-Laure Fameau, Marcus Gast, Heni Dallagi, and Sunghwan Jung are thanked for exchanges during the preparation of the manuscript. Funding is acknowledged from the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt, DBU, AZ 34788/01-45), from

the Institute of Advanced Studies at the University of Strasbourg (USIAS), and from European Research Council (ERC consolidator grant, agreement 819511 - METAFOAM).

## References

Papers of particular interest, published within the period of review, have been highlighted as:

* of special interest

** of outstanding interest

1. Waternews Europe, Europe: Increase of water regulations car wash industry. https://www.waternewseurope.com/europe-increase-of-water-regulations-car-wash-industry/. Accessed 8 April 2024.

2. Wang Q, Huang N, Cai H, Chen X, Wu Y: Water strategies and practices for sustainable development in the semiconductor industry. *Water Cycle* 2023, 4:12–16.

3. Mousavi SA, Khodadoost F: Effects of detergents on natural ecosystems and wastewater treatment processes: a review. *Environ Sci Pollut Res* 2019, 26:26439–26448.

Systematic work to elucidate the velocity-dependent mechanisms.

4. Haushaltspflege, IKW-Nachhaltigkeitsbericht aktuell, https://www.ikw.org/haushaltspflege/themen/detail/ikw-nachhaltigkeitsbericht-aktuell/(accessed 8 April 2024).

5. Peet M, Robinson D: Leading questions, Nelson, Walton-on-Thames. 1992.

6. Jones S, Rio E, Cazeneuve C, Nicolas-Morgantini L, Luengo GS: Tribological influence of a liquid meniscus in human sebum cleaning. *COLSUA* 2016, 498:268–275.

7. Andreev VA, Prausnitz JM, Radke CJ: Meniscus-shear particle detachment in foam-based cleaning of silicon wafers with an immersion/withdrawal cell. *Ind Eng Chem Res* 2010, 49: 12461–12470.

One of the first investigations of foam-based cleaning.

8. Schad T, Preisig N, Blunk D, Piening H, Drenckhan W,

**: Stubenrauch C: Less is more: unstable foams clean better than stable foams. *JCIS* 2021, 590:311–320.

First proof of the efficiency of unstable foams for cleaning purposes due to the dynamics created by bubble reorganisation.

9. Dallagi H, Faille C, Bouvier L, Deleplace M, Dubois T, Aloui F, *: Benezech T: Wet foam flow: a suitable method for improving surface hygiene in the food industry. *J Food Eng* 2022, 322, 110976.

First interpretation of foam-flow cleaning efficiency via explicit measurement of wall shear stresses – and their fluctuations.

10. Weaire D, Hutzler S: *The physics of foams*. Oxford: Clarendon Press; 1999.

11. Youtube: *What is ecobubble washing machine technology | Samsung core tech | Samsung UK*. https://www.youtube.com/watch?v=myh327hIA4k. Accessed 8 April 2024. https://news.samsung.com/global/how-ecobubble-works.

12. Guzmán García Lascurain P, Goidanich S, Briatico Vangosa F, Anzani M, Rabbolini A, Sansonetti A, Toniolo L: *Agar foam: properties and cleaning effectiveness on gypsum surfaces*. *Coatings* 2023, 13:615, https://doi.org/10.3390/coatings13030615.

13. Geyer F, D'Acunzi M, Sharifi-Aghili A, Saal A, Gao N, Kaltbeitzel A, Sloot T-F, Berger R, Butt H-J, Vollmer D: *When and how self-cleaning of superhydrophobic surfaces works*. *Sci Adv* 2023, 6, eaaw9727.

14. Tuteja A, Choi W, Mabry JM, Cohen RE: Robust omniphobic surfaces. *Proc Natl Acad Sci USA* 2008, 105:18200–18205.

15. Li P, Wang J, Liao Z, Ueda Y, Yoshikawa K, Zhang G: Microbubbles for effective cleaning of metal surfaces without chemical agents. *Langmuir* 2022, 38:769–776.

16. Gómez Suárez C, Buscher HJ, van der Mei HC: Analysis of bacterial detachment from substratum surfaces by the passage of air-liquid interfaces. *Appl Environ Microbiol* 2001, 67:2531–2537, https://doi.org/10.1128/AEM.67.6.2531-2537.2001.
17. Menesses M, Belden J, Dickenson N, Bird J: Measuring a critical stress for continuous prevention of marine biofouling accumulation with aeration. *Biofouling* 2017, **33**: 703–711.

18. Bullard SG, Shumway SE, Davis CV: The use of aeration as a simple and environmentally sound means to prevent biofouling. *Biofouling* 2010, **26**:587–593.

19. Schad T, Preisig N, Piening H, Stubenrauch C: Innovative foam-based cleaning concepts for historical objects. *Tenside Surf. Det.* 2022, 59:451–459.

20. Denkov ND, Subramanian V, Gurovich D, Lips A: Wall slip and viscous dissipation in sheared foams: effect of surface mobility. *Colloids Surfaces A Physicochem. Eng. Asp.* 2005, **263**: 129.

21. Cantat I, Cohen-Addad S, Elias F, Graner F, Höhler R, Pitois O, Rouyer F, Saint-Jalmes A: Foams - structure and dynamics. Oxford, UK: Oxford University Press; 2013.

22. Saugey A, Drenckhan W, Weaire D: Wall slip of bubbles in foams. *Phys Fluids* 2006, **18**, 053101.

23. Raufaste C, Foulon A, Dollet B: Dissipation in quasi-two-dimensional flowing foams. *Phys Fluids* 2009, **21**, 053102.

24. Cantat I: Liquid meniscus friction on a wet plate: bubbles, lamellae, and foams. *Phys Fluids* 2013, **25**.

Excellent clarification of friction created by liquid interfaces with different interface mobilities.

25. Marchand M, Restagno F, Rio E, Boulogne F: Roughness-induced friction on liquid foams. *Phys Rev Lett* 2020, **124**, 118003.

26. Denkov ND: Mechanisms of foam destruction by oil-based antifoams. *Langmuir* 2004, **20**:9463–9505.

27. Mensire R, Piroird K, Lorenceau E: Capillary imbibition of aqueous foams by miscible and nonmiscible liquids. *Phys Rev E* 2015, **92**, 053014.

28. Mensire R, Ault JT, Lorenceau E, Stone HA: Point-source imbibition into dry aqueous foams. *EPL* 2016, **113**, 44002.

29. Leenaars AFM, O'Brien SBG: Particle removal from silicon substrates using surface tension forces. *Philips J Res* 1989, **44**:183.

30. O'Brien SBG, Van Den Brule BHA: A mathematical model for the cleansing of silicon substrates by fluid immersion. *J Colloid Interface Sci* 1991, **144**:210–221.

31. Noordmans J, Wit PJ, Van Der Mei HC, Busscher HJ: Detachment of polystyrene particles from collector surfaces by surface tension forces induced by air-bubble passage through a parallel plate flow chamber. *J Adhes Sci Technol* 1997, **11**:957–969, https://doi.org/10.1163/156856197X00525.

32. Gómez Suárez C, Noordmans J, van der Mei HC, Busscher HJ: Removal of colloidal particles from quartz collector surfaces as stimulated by the passage of Liquid-Air interfaces. *Langmuir* 1999, **15**:5123–5127.

33. Gómez Suárez C, Noordmans J, van der Mei CH, Busscher HJ: Detachment of colloidal particles from collector surfaces with different electrostatic charge and hydrophobicity by attachment to air bubbles in a parallel plate flow chamber. *Phys Chem Chem Phys* 1999, **1**:4423–4427.

34. Lazouskaya V, Jin Y: Colloid retention at air-water interface in a capillary channel. *Colloids Surfaces A Physicochem. Eng. Asp.* 2008, **325**:141–151.

35. Sharma P, Flury M, Zhou J: Detachment of colloids from a solid surface by a moving air-water interface. *J Colloid Interface Sci* 2008, **326**:143–150.

36. Kondoyan A, Dessaigne S, Herry J-M, Bellon-Fontaine M-N: Capillary force required to detach micron-sized particles from solid surfaces—validation with bubbles circulating in water and 2µm-diameter latex spheres. *Colloids Surf B Bio-interfaces* 2009, **73**:276–283.

37. Aramrak S, Flury M, Harsh JB: Detachment of deposited colloids by advancing and receding air-water interfaces. *Langmuir* 2011, **27**:9985–9993.

38. Zevi Y, Gao B, Zhang W, Morales VL, Ekrem Cakmak M, Medrano EA, Sang W, Steenhuis TS: Colloid retention at the meniscus-wall contact line in an open microchannel. *Water Res* 2012, **46**:295–306.

39. Aramrak S, Flury M, Harsh JB, Zollars RL, Davis HP: Does colloid shape affect detachment of colloids by a moving air-water interface? *Langmuir* 2013, **29**:5770–5780, https://doi.org/10.1021/la400252q. 2013.

40. Lazouskaya V, Wang L-P, Or D, Wang G, Caplan JL, Jin Y: Colloid mobilization by fluid displacement fronts in channels. *J Colloid Interface Sci* 2013, **406**:44–50.

41. Naga A, Kaltbeitzel A, Wong WSY, Hauer L, Butt H-J, Vollmer D: How a water drop removes a particle from a hydrophobic surface. *Soft Matter* 2021, **17**:1746–1755, https://doi.org/10.1039/D0SM01925A.

Detailed and systematic work to elucidate the microscopic detachment mechanisms.

42. Khodaparast S, Kim MK, Silpe JE, Stone HA: Bubble-driven detachment of bacteria from confined microgeometries. *Environ Sci Technol* 2017, **51**:1340–1347, https://doi.org/10.1021/acs.est.6b04369.

43. Gómez-Suárez C, Busscher Henk J, van der Mei Henny C: Analysis of bacterial detachment from substratum surfaces by the passage of air-liquid interfaces. *Appl Environ Microbiol* 2001, **67**:2531–2537.

44. Thobie C, Blel W, Hauser J-L, Pruvost J, Gentric C: Different types of bubbly flows in a confined channel with the aim of limiting microalgae biofilm development - Part I: hydrodynamic study. *Chem. Eng. Process. - Process Intensif.* 2022, **173**, 108844.

45. Thobie C, Blel W, Dupré C, Marec H, Pruvost J, Gentric C: Different types of bubbly flows in a confined channel with the aim of limiting microalgae biofilm development - Part II: study of the development and removal of microalgae biofilm. *Chem. Eng. Process. - Process Intensif.* 2022, **175**, 108899.

46. Khodaparast S, Boulogne F, Poulard C, Stone HA: Water-based peeling of thin hydrophobic films. *Phys Rev Lett* 2017, **119**, 154502.

47. Parini MR, Pitt WG: Dynamic removal of oral biofilms by bubbles. *Colloids Surf B Biointerfaces* 2006, **52**:39–46, https://doi.org/10.1016/j.colsurfb.2006.06.005.

48. Lowen BJ, Deibel D, McKenzie CH, Couturier C, DiBacco C: Tolerance of early life-stages in Ciona intestinalis to bubble streams and suspended particles. *Manag. Biol. INVASIONS* 2016, **7**:157–165.

49. Esmaili E, Shukla P, D Eifert J, Jung S: Bubble impact on a tilted wall: removing bacteria using bubbles. *Phys. Rev. Fluids* 2019, **4**, 43603.

50. Jang H, Rusconi R, Stocker R: Biofilm disruption by an air bubble reveals heterogeneous age-dependent detachment patterns dictated by initial extracellular matrix distribution. *npj Biofilms Microbiomes* 2017, **3**:6, https://doi.org/10.1038/s41522-017-0014-5.

51. Hooshanginejad A, Sheppard T, Xu P, Manyalla J, Jaicks J, Esmaili E, Jung S: Effect of angle in removing proteins or bacteria on a tilted surface using air bubbles. *Physical Review Fluids* 2023, **8**, 43602.

Clean and systematic work to identify the key mechanisms acting to remove biofilms.

52. Scardino AJ, Fletcher LE, Lewis JA: Fouling control using air bubble curtains: protection for stationary vessels. *J. Mar. Eng. Technol.* 2009, **8**:3–10.

53. Hopkins GA, Gilbertson F, Floerl O, Casanovas P, Pine M, Cahill P: Continuous bubble streams for controlling marine biofouling on static artificial structures. *PeerJ* 2021, **9**, e11323.
54. Miyamoto M, Ueyama S, Hinomoto N, Saitoh T, Maekawa S, Hirotsuji J: **Degreasing of solid surfaces by microbubble cleaning**. *Jpn J Appl Phys* 2007, **46**:1236, https://doi.org/10.1143/JJAP.46.1236. 2007.

55. Mensire R, Lorenceau E: **Stable oil-laden foams: formation and evolution**. *ACIS* 2017, **247**:465–476.

Excellent review on oil-uptake by foams.

56. Kusaka A, Sonoda J, Tajima H, Sakai T: **Dynamics of liquid oil that flows inside aqueous wet foam**. *J Phys Chem B* 2018, **122**: 9786–9791.

Extension of oil imbibition into foams with non-negligible liquid fractions.

57. Schad T, Preisig N, Drenckhan W, Stubenrauch C: **Foam-based cleaning of surfaces contaminated with mixtures of oil and soot**. *J Surfactants Deterg* 2022, **25**:377–385.

First investigation of soot uptake by foams with the help of oil imbibition.

58. Dari C, Cousin F, Le Coeur C, Dubois T, Bénézech T, Saint-Jalmes A, Fameau A-L: **Ultrastable and responsive foams based on 10-hydroxystearic acid soap for spore decontamination**. *Molecules* 2023, **28**:4295.

59. Pugh RJ: **Bubble and foam chemistry**. Cambridge University Press; 2016.

60. Langevin D: *Emulsions, microemulsions and foams, series: soft and biological matter*. Springer Nature Switzerland; 2020.

61. Piroird K, Lorenceau E, Biance A-L: **Oil repartition in a foam film architecture**. *Soft Matter* 2014, **10**:7061–7067.

62. Tisné P, Aloui F, Doubliez L: **Analysis of wall shear stress in wet foam flows using the electrochemical method**. *Int J Multiphas Flow* 2003, **29**:841–854.

63. Tisné P, Doubliez L, Aloui F: **Determination of the slip layer thickness for a wet foam flow**. *Colloids Surf A Physicochem Eng Asp* 2004, **246**:21–29.

64. Al Saabi A, Dallagi H, Aloui F, Faille C, Rauwel G, Wauquier L, Bouvier L, Bénézech T: **Removal of Bacillus spores from stainless steel pipes by flow foam: effect of the foam quality and velocity**. *J Food Eng* 2021, **289**, 110273.

65. Dallagi H, Faille C, Gruescu C, Aloui F, Benezech T: **Foam flow cleaning, an effective and environmentally friendly method for controlling the hygiene of closed surfaces contaminated with biofilms**. *Food Bioprod Process* 2022, **136**:236–248.

66. Dari C, Dallagi H, Faille C, Dubois T, Lemy C, Deleplace M, Abdallah M, Gruescu C, Beaucé J, Bénézewch T, Fameau A-L. **Decontamination of spores on model stainless-steel surface by using foams based on alkyl polyglucosides**. *Molecules* 2023, **28**:936.

67. Piroird K, Lorenceau E: **Capillary flow of oil in a single foam microchannel**. *Phys Rev Lett* 2013, **111**, 234503.

68. Sonoda J, Sakai T, Inomata Y: **Liquid oil that flows in spaces of aqueous foam without defoaming**. *J Phys Chem B* 2014, **118**: 9438–9444.

69. Schramm LL, Novosad JJ: **Micro-visualization of foam interactions with a crude oil**. *Colloid Surface* 1990, **46**:21–43.

70. Gaillard T, Roché M, Honorez C, Jumeau M, Balan A, Jedrzejczyk C, Drenckhan W: **Controlled foam generation using cyclic diphasic flows through a constriction**. *Int J Multiphas Flow* 2017, **96**:173–187.

71. Steck K, Hamann M, Andrieux S, Muller P, Kéckicheff P, Stubenrauch C, Drenckhan W: **Fluorocarbon vapors suppress coalescence in foams**. *Adv Mater Interfac* 2021, **8**, 2100723.

72. Steck K, Dijoux J, Preisig N, Bouylout V, Stubenrauch C, Drenckhan W: **Fluorocarbon vapors slow down coalescence in foams: influence of surfactant concentration**. *Colloid Polym Sci* 2023, **301**:685–695.

73. Boulogne F, Rio E, Restagno F: **Evaporation-induced temperature gradient in a foam column**. *Langmuir* 2023, **39**: 14256–14262.

Original work highlighting the importance of evaporation-based cooling for soap films

74. Deleurence R, Saison T, Lequeux F, Monteux C: **Time scales for drainage and imbibition in gellified foams: application to decontamination processes**. *Soft Matter* 2015, **11**:7032.