# Auto-Emulsification of Water at the Crude Oil/Water Interface: Role of the Solid Walls and Wettability Alteration

Duala Puentes-Rivas, Alexandra Klimenko, Maurice Bourrel, François Lequeux, Pascal Panizza

► To cite this version:

Duala Puentes-Rivas, Alexandra Klimenko, Maurice Bourrel, François Lequeux, Pascal Panizza. Auto-Emulsification of Water at the Crude Oil/Water Interface: Role of the Solid Walls and Wettability Alteration. Energy & Fuels, 2025, 39 (25), pp.12125-12138. 10.1021/acs.energyfuels.5c01691. hal-05165410

HAL Id: hal-05165410

https://univ-rennes.hal.science/hal-05165410v1

Submitted on 16 Jul 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Auto-Emulsification of Water at the Crude
Oil/Water Interface: Role of the Solid Walls and
Wettability Alteration

Duala Puentes-Rivas,†,‡ Alexandra Klimenko,†,‡ Maurice Bourrel,†,‡ François

Lequeux,†,¶ and Pascal Panizza*,§,¶

†Laboratoire Physico-Chimie des Interfaces Complexes, Bâtiment CHEMSTARTUP, RD
817, 64170 Lacq, France

‡TotalEnergies S.E., Pôle d'Etude et de Recherches de Lacq, BP 47, 64170 Lacq, France

¶Laboratoire Sciences et Ingénierie de la Matière Molle, ESPCI Paris, PSL University,
Sorbonne Université, CNRS UMR 7615, 75005 Paris, France

§IPR, UMR CNRS 6251, Campus Beaulieu, Université Rennes 1, 35042 Rennes, France

E-mail: pascal.panizza@univ-rennes.fr

Abstract

Using confocal fluorescence microscopy in both hydrophilic and hydrophobic glass capillaries, this study explores (i) the influence of wall wettability on spontaneous emulsification at the oil/water interface and (ii) the mechanisms governing the interaction of W/O microdroplets with solid surfaces, ultimately enhancing oil recovery during low-salinity water injection. Our findings reveal that microdroplet formation predominantly occurs on solid surfaces when they are hydrophilic, whereas in hydrophobic environments, droplets emerge at the water/oil interface—specifically within the thin film between the meniscus and the solid wall. The growth dynamics of these droplets
follow distinct scaling laws depending on the wettability of the surface. Upon injecting low-salinity water, the apparent contact angle increases on hydrophilic walls but remains unchanged on hydrophobic ones. In both cases, however, the advancing meniscus leaves behind a residual oil film containing trapped water microdroplets. The mobility of these droplets—whether sessile or not—depends on surface properties. Over time, they merge with the surrounding water, forming water patches within the residual oil film. As these patches expand and coalesce, they create a structure reminiscent of a two-dimensional foam. Ultimately, these patches continue to grow until the solid wall is fully exposed to the free water phase.

# 1. Introduction

Enhanced Oil Recovery (EOR) techniques have become critical in extending the productive life of oil reservoirs,¹,² especially after the depletion of primary and secondary recovery methods. Among these, Low Salinity Waterflooding (LSW) has emerged as a promising technique due to its economic feasibility and significant potential for improving oil recovery. First introduced by Bernard in 1967,³ and later developed by N. Morrow's group,⁴,⁵ LSW involves the injection of water with reduced salinity, which has been shown to alter the interactions between brine, oil, and rock surfaces, ultimately improving oil displacement.⁶ Since Bernard's pioneering work, extensive research has been conducted to elucidate the mechanisms driving LSW's effectiveness.

Besides, over the last decade, the research community has reached the consensus that, to extract adsorbed oil from the rock matrix, LSW injection evolves the wettability to a less oil-wet state, more favorable for oil recovery.⁷,⁸ Interfacial tension reduction has been considered as playing an important role.⁹ Electrostatic phenomena at the oil/brine interface, double-layer expansion, changes in surface charge and hydration effects are found to be critical in this process.¹⁰Recent results have shown that sequential dilution of water salinity significantly improves oil recovery, underlining the importance of managing water salinity to
optimize EOR strategies.⁸

Among the various envisioned mechanisms, it was found that a mechanism involving self-emulsification or spontaneous formation of water-in-oil (W/O) microdroplets at the water/oil interface could be involved in wettability modification and thus play a role in enhancing oil recovery during low-salinity water flooding. This so-called microdispersion process has been found to operate both in sandstone and carbonate reservoirs, leading to improved oil recovery.¹¹⁻¹³ It has been shown that this spontaneous formation and growth of microdroplets is driven by the osmotic pressure gradient which takes place in the presence of polar compounds in crude oils, such as asphaltenes.¹⁴ However, the intimate connection between microdroplets formation in the oil and mineral surface wettability change remains to be established.

Additionally, it is worth underlining the swelling of water films driven by osmotic pressure gradient between high and low salinity water regions, which induces water migration, leading to remobilization of oil trapped in a pore. The oil acts as a semi-permeable membrane, allowing water to migrate while blocking ions, thereby creating a pressure differential that drives oil displacement. This process is especially accelerated in the presence of asphaltenes, where the formation of microdispersion was also noted.¹⁵ Studies using functionalized glass micromodels,¹²,¹⁶ fractured silica systems¹⁷,¹⁸ and carbonate rocks¹⁹,²⁰ have provided detailed insights into the pore-scale mechanisms that govern the formation and growth of water films on solid surfaces initially covered with oil. When crude oil was used, the swelling of the water film was accompanied by the formation of microdispersions, but the potential relationship between these phenomena has yet to be established.

From this extensive body of research, it can be inferred that the combined effects of self-emulsification, interfacial tension reduction, and wettability alteration work synergistically to enhance oil displacement efficiency during low salinity waterflooding. In this study, we first explore the mechanisms driving auto-emulsification at the interface between an asphaltene-laden oil phase and a slightly saline water phase, especially at the contact with a mineral
surface. We then investigate the intimate mechanism by which the appearance of W/O microdroplets affects the wettability of hydrophilic and hydrophobic solid surfaces, in both static and dynamic conditions.

# 2. Materials and Methods

## 2.1 Materials

The model oil is a solution of toluene (Sigma-Aldrich, HPLC, 99.9%) containing 1% weight fraction of asphaltenes. Asphaltenes are extracted from a Venezuelan bituminous sand by precipitation with n-heptane (AnalaR NORMAPUR®, 99.8%) and subsequently purified without Soxhlet extraction. The solution of asphaltanes in toluene is left to stand for at least 12 hours to ensure complete dissolution of solid materials and equilibrium is reached with the solvent. The water solution is an aqueous solution comprising 0.01 g/L NaCl, prepared with 99.5% sodium chloride (AnalaR NORMAPUR® ACS) and deionized water obtained from a Millipore water purification system (Direct-Q 3UV, Merck). The resistivity of the deionized water is 18 MΩ.cm. All experiments are performed at room temperature.

## 2.2 Method

### Preparation of capillaries:

The rectangular capillaries (vitrotubes) used in our experiments are made of borosilicate glass, measuring 50 mm in length, 4 mm in width, 0.4 mm in height, and having a wall thickness of 0.28 mm.

*Chemical treatment:* A hydrophilic treatment is first applied to ensure a clean, homogeneous surface that is perfectly wettable by water. The capillaries are washed with distilled water and ethanol, then cleaned with a low-pressure plasma system (Plasma Surface Technology Diener, FEMTO) for 10 minutes. This step removes impurities and contaminants from the surface. Next, the capillaries are left in contact with a 1 M NaOH solution (for
around 2 hours) to ensure that the surface is perfectly hydrophilic. They are then washed
with deionized water to remove excess NaOH and dried overnight in an oven at 90° C.
Dried capillaries are then stored in a glass desiccator. Once these hydrophilic capillaries
have been prepared, they can be treated by silanization to render them hydrophobic. To do
this, the hydrophilic capillaries are filled with a solution of chloro(dimethyl)octadecylsilane
solubilized in 0.5% *wt* dichloromethane and placed in a hermetically sealed bottle for at least
12 hours. They are then washed with dichloromethane (DCM) to remove excess unreacted
silane. Finally, they are oven-dried overnight. Before use, they are passed under a stream of
*N*₂ gas to remove excess DCM and air and minimize moisture on the walls.

*Set-up:* At each end of the capillary, two 4 to 5 mm long molten silicone needles are inserted to allow the entry and exit of the oil and water phases. For inlets and outlets, the internal diameters chosen for these needles are 0.075 mm and 0.150 mm respectively. Their external diameter is 0.360 mm. The ends of the needles were previously inserted into 1/16" OD x 0.076 mm ID PTFE plastic tubes. The ends of each PTFE tube are inserted into 1/16" low-pressure shut-off valves. The tubes are subsequently connected to the corresponding valves, which lead to the inlet and outlet syringes that are already connected to a CETONI Base 600 syringe pump. Both sides of the capillary are attached to a glass slide with Scotch-Weld DP100 clear adhesive that is placed on the stage of a Leica DMi8 inverted microscope.

*Capillary filling:* Before filling the capillary with the two liquid phases, it is first purged with Nitrogen for 30 minutes to limit the presence of water (moisture) on its walls. A 2.5 mL glass syringe (SGE, PTFE tip) is used to inject the aqueous phase. Injection is performed with the syringe pump at a controlled flow rate, typically between 0.1 and 0.2 µL/s, with the capillary placed vertically. Before the injection of aqueous phase, Φ<sub>W</sub>, the valve is first purged to eliminate any air bubbles. During this purge, the aqueous phase is directed into a waste bottle via a single-headed 4-way PEEK T-valve as depicted in Fig. 1. At the start of the injection phase, the valve connected to the outlet tube is left open, and the fluid injected through the inlet port is then drawn into the outlet port at the same rate
as the injection flow, thanks to another syringe pump connected to the outlet port. When both inlet and outlet tubes are filled with Φ<sub>W</sub>, the valve connected to the outlet tube is then closed and the injection phase continues until the capillary is filled halfway (the volume of Φ<sub>W</sub> injected into it is then ≈ 40μL). The valve connected to the inlet tube is then closed. The oil phase, Φ<sub>O</sub>, is then injected into the device at a controlled flow rate of between 0.1 and 0.2 μL/s using a 500 μL glass syringe (Hamilton) mounted on a syringe pump. As explained above, the first step is to purge the oil phase inlet valve to eliminate the presence of any air bubbles. To do this, the injected oil phase is directed into a waste bottle via a 4-way single-head PEEK T-valve (see Fig. 1). It is then injected into the capillary at a set flow rate of between 0.1 and 0.2 μL/s, until the capillary is completely filled. As the outlet tube begins to fill, the inlet flow rate is lowered to 0.05 μL/s to help eliminate small air bubbles forming at the end of the outlet needle. Filling of the outlet tube continues at a flow rate of 0.2 μL/s until the oil phase reaches the outlet syringe. The inlet and outlet valves are then closed and the capillary placed horizontally on the microscope stage.

Microscopic Observations:

3D microscopic observations are made using a confocal microscope, specifically a Leica
TCS SP8 scanning head and a Leica DMi8 inverted microscope stand, with LAS X Navigator
image acquisition software configured in resonant mode. The excitation wavelength of the
laser, chosen at 552 nm (visible), is close to and greater than that corresponding to the
absorption peak of the natural fluorophores present in the oil.²¹ For all our experiments,
an HC PL APO 10x/0.40 CS objective is used with a depth of field around 0.5 µm. The
entire thickness of the capillary is observed at different heights (Z axis), and cross-sections
of the oil/water interface are obtained in the XY plane. When used in fluorescence mode,
the microscope generates XYZ fluorescence intensity variation data as a function of time,
enabling a 3D image to be reconstructed from a stack of XYZ images at any time, and
vertical sections of the sample to be drawn in different XZ or YZ planes. A 3D image of the
<figure><img src="image_2.png" /><figcaption>Figure 1: Diagram showing the capillary filling device and the various stages involved in filling the capillary tube with the two fluids and bringing them into contact with each other. For each step, the state of the various valves (closed or open) is indicated.</figcaption></figure>
meniscus can thus be extracted over time. In parallel, a transmission microscope with the
same objective is used to visualize the upper and lower walls of the capillary near the triple
line separating the oil phase from the water phase observed as shown in Fig. 2. This enables
us to characterize the kinetic evolution of the W/O microdroplets that spontaneously form.

<figure><img src="image_3.png" /><figcaption>Figure 2: Diagram showing the shape of the interface separating the two imiscible phases in the case of a hydrophobic surface A) and a hydrophilic surface B). The different planes of observation of the fluid in the cell are schematised: Panels U and L correspond respectively to the upper and lower surfaces of the capillary in contact with the fluids. Panel M corresponds to the observation of the fluid layer located on the median plane of the cell.</figcaption></figure>

**3. Results**

**3.1 W/O Microdroplet Formation Dynamics**

First, we study the self-emulsification process under static conditions, i.e. when the two phases are brought into contact and left at rest. The aim is to assess the influence of the nature of the solid surfaces on this formation/emulsification process.

**3.1 a Hydrophilic Solid Surfaces**

First, the case where the solid surfaces are hydrophilic is studied. Fig. 3 a is a schematic view of the top of the capillary, enabling the location of the zone of microscopic observations (black box) in relation to the initial triple line position, corresponding to the contact between the two fluids at $t=0$ (green dotted line). The solid green line corresponds to the position of the triple line at time $t$, taken after the two phases have come into contact. It is interesting to note that the self-emulsification process discussed afterwards is sometimes accompanied
by a slight displacement of the triple lines on the upper (U) and lower (L) surfaces, which
is not reproducible from one experiment to another. Comparing the continuous and dotted
green lines allows for following the evolution of the triple line.

Fig. 3 b and d show images taken with the confocal laser scanning microscope trans-
mission mode, on the (U) and (L) surfaces of the capillary respectively at time t. Once the
two phases have been brought into contact, W/O microdroplets spontaneously form on the
capillary walls, gradually increasing in volume over time and potentially coalescing when
they touch, with no significant differences observed between surfaces (U) and (L). Prefer-
ably, these droplets form on the hydrophilic surface at a distance relatively close to the initial
triple line, growing and coalescing more rapidly the closer they are to the meniscus.

Figs. 3 c and e show enlarged images of the images taken in the red and blue boxes of
Figs. 3 b and d respectively. These images were analysed with the Image J FIJI software,
in order to follow the evolution kinetics of the microdroplets on surfaces (U) and (L). This
analysis was carried out from 5 hours after the contact between the contact of the two
phases. Before this time, the droplets, if present, are too small in size to be detected due to
the resolution of our set-up (< 1.5 µm).

Fig. 4 shows the evolution of the mean Feret diameter *d*(*t*) and the number of micro-
droplets *N*(*t*) over time, respectively, for the (U) and (L) surfaces after contact of the contact
of 2 phases. The graph obtained for *d*(*t*) shows that the mean Feret diameter of the droplets
observed on the two surfaces follows a power law over time: *d* ∝ *t*¹/³ with the similar size of
the droplets on the two surfaces. The graph obtained for *N*(*t*) shows the existence of two
distinct power law regimes *N* ∝ *t*ª. The value of the coefficient α is 3 for the regime I and
−1/2 for the regime II.

In regime I, small water nuclei nucleate at the surface of the hydrophilic capillary and
then grow. The droplets formed are initially small (< 1.5 µm) and far apart from each
other. The value of the exponent 1/3 is similar that found for the growth of droplet governed
by Oswald Ripening.²²,²³ This suggests that the growth of each droplet is primarily driven by
<figure><img src="image_4.png" /><figcaption>Figure 3: Spontaneous formation of W/O microdroplets occurring after contact of the 2 phases in an hydrophilic capillary a) Top view of the capillary filled with the two phases indicating the field of our microscopic observations and its position relative to the initial triple line (green dotted line). The continuous green represents the position of the triple line at time *t* after the contact between both phases (it is moving slightly). Φ<sub>W</sub>, the water phase is an aqueous solution containing 0.01 g/L NaCl and Φ<sub>O</sub> is a toluene solution in which 1% by mass of asphaltenes have been solubilised. b) and d) Transmission images of the upper (U) and lower (L) surfaces of the capillary acquired at different times *t*. c) and e) Analysis of the enlarged images corresponding to the areas defined by the red and blue boxes in images b) and d), respectively.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4: Time trends in the number of droplets *N* and their mean Feret diameter *d* detected in the red and blue boxes in Figs. 3b and d). The data are obtained by processing and analysis of the images shown in Figs. 3c and e). The red and blue symbols correspond to the data extracted from the images taken on surfaces U (top wall) and L (lower wall), respectively.</figcaption></figure>

the diffusive transport of water molecules from the free water phase. However, a secondary
contribution from the transfer of water from a population of smaller, non-visible droplets-
facilitated by their higher chemical potential due to Laplace pressure- cannot be excluded.
The number of detected droplets varies as $N(t) \propto t^3$, indicating the nucleation rate $\frac{dN}{dt} \propto t^2$,
until a maximum number of droplets is reached, once they come into contact with each other.

In regime II, approximately 12 hours after contact between the two phases, the droplets
that are now in contact with each other begin to coalesce (see Fig. 5), leading to a decrease in
their number. The coalescing droplets form non-circular water pockets due to local defects
on the wall. Observations made in the median plane of the capillary did not reveal the
formation of W/O droplets in the bulk oil phase.
<figure><img src="image_6.png" /><figcaption>Figure 5: Transmission microscopic images showing the evolution of the number of droplets on the upper wall surface of an hydrophilic capillary in regime I. The area highlighted in blue shows that the growth of an isolated drop is ensured by a molecular supply of water by osmotic transfer. The area highlighted in red shows that coalescence may occur between sessile drops when they touch each other.</figcaption></figure>
### 3.1 b Hydrophobic Solid Surfaces

We are now studying the self-emulsification process of W/O microdroplets when the capillary walls are hydrophobic. In contrast to the previous case (hydrophilic walls), transmission microscopic observations made in the median plane of the capillary show the formation and growth of numerous microdroplets in this plane at the W/O interface. The distribution of these droplets appears to be monodisperse throughout the process, with the droplets forming a compact assembly (see Fig. 6). Their average diameter increases linearly with time ($d \propto t$). Because of this large thickness, we were unfortunately unable to measure the evolution of the thickness of the microdroplet layer in the median plane. Duboue et al. have observed in her experiments that this thickness varied in $\propto t^{1/2}$.¹⁴

We also followed the kinetics of microdroplet formation on the lower capillary wall using transmission microscopy images (see Fig. 7). Very quickly after the two fluids come into contact, we observe the formation and growth of W/O microdroplets. In the course of time, new droplets appear while others grow in size by the molecular input of water. Fig. 7 c shows the temporal evolutions of the number of droplets, $N(t)$, and their mean Feret diameter, $d(t)$. For the droplets located at the (L) surface, $d(t)$ increases linearly with time, similarly to the behaviour observed for droplets suspended within the bulk (see Fig. 6). Fig. 8 shows the evolution of the thin oil film connecting the meniscus to the triple line. After their appearance, when W/O microdroplets come into contact with each other, they form aggregates without coalescing, indicating the existence of adhesive forces between them. This also suggests that coalescence of sessile droplets (as observed in the previous section) is easier than that of suspended droplets, and therefore that the triple line facilitates coalescence. As shown in Fig. 8 d-f, the images acquired in fluorescent mode show the shape of the meniscus and the temporal evolution of the microstructure of the oil film underneath. We observe that the meniscus slightly moves towards the area of the capillary occupied by the macroscopic oil and that the film thins. Visualisation of vertical sections shows that the micro-droplets of water that form within it grow and can coalesce with the macroscopic
water, forming water patches that pierce the oil film.

<figure><img src="image_7.png" /><figcaption>Figure 6: a) Transmission images taken in the median plane of the hydrophobic capillary, illustrating the kinetic formation of W/O microdroplets in this plane. b) Evolution over time of the mean diameter of the microdroplets. The time origin corresponds to the contact between the two phases. The solid line corresponds to the best linear fit. Its equation is $d(\mu m)=1.67t(h)$.</figcaption></figure>

## 3.2 Impact of O/W Microdroplets Presence on Meniscus Displacement during Low Salt Water Injection

After several hours of contact between the two phases, during which we monitored and characterised the self-emulsification process, we are now interested in the movement of the meniscus when low salt water is injected into the device, thus reproducing what happens during water injection in a field. During this injection phase, we monitor the structure of the
<figure><img src="image_8.png" /><figcaption>Figure 7: a) Top view of the capillary filled with the two phases indicating the field of our microscopic observations and its position relative to the triple line (green dotted line) and to the meniscus (yellow line). b) Transmission views at different times of the lower wall of the hydrophobic capillary when the two phases are brought into contact. c) Time trends in the number of droplets *N* and their mean Ferret diameter *d* detected in the blue box shown in b).</figcaption></figure>
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

Figure 8: a) Top view of the capillary filled with the two phases indicating the field of our microscopic observations and its position relative to the triple line (blue dotted line) and to the meniscus (yellow line). b) and c) are transmission views at different times of the lower wall of the hydrophobic capillary when the two phases are brought into contact. As the capillary is hydrophobic, an oil film of varying thickness exists between these two positions. Within this film, we note the continuous appearance of slowly growing W/O micro-droplets. d) Images acquired in fluorescence mode of the regions corresponding to the blue boxes shown in c). The dotted pink and orange lines indicate the positions of the two vertical slice planes obtained by 3D reconstruction of the stack of XYZ images acquired at the same times and shown in e) and f) respectively. The presence of a drop trapped in the oil film and a hole (i.e., a water patch) are indicated by arrows. Figure f) shows the coalescence between two holes.
fluid on the capillary surfaces located on either side of the meniscus. As before, we investigate
the case of hydrophilic and hydrophobic surfaces. In all our experiments—whether conducted
with hydrophobic or hydrophilic capillaries of rectangular cross-section (height h=400 µm,
width w=4 mm)—the aqueous phase was injected at a constant flow rate of Q=0.33 µL/h.
This corresponds to an average injection velocity of approximately V=$\frac{Q}{hw}$=0.06 µm/s and
a shear rate of $\dot{\gamma} \simeq \frac{2V}{h}$ = 3.10$^{-4}$ s$^{-1}$. In oil fields, the typical average injection rate is
approximately 30 cm per day. For a reservoir with a porosity of 30% and a characteristic
pore size of d ≃ 25 µm, this translates to an effective pore-scale injection velocity of about
12 µm/s-roughly 200 times higher than the velocity used in our experiments and a shear
rate of roughly 2 s$^{-1}$, which is approximately 10$^{4}$ times greater than the shear rate applied
in our study. Such discrepancies in velocity and shear rates are, in fact, quite common
between laboratory-scale studies and field conditions. Nevertheless, given the pronounced
structural heterogeneity of reservoirs-characterized by numerous fractures that serve as pri-
mary flow conducts-it is reasonable to assume that the velocity employed in our experiments
is comparable in order of magnitude to those occurring in the non-preferential flow paths
within the porous matrix which play a key role in oil recovery. It is therefore quite likely
that the phenomena observed in our study are relevant to the processes happening under
field conditions.

3.2 a Hydrophilic Solid Surfaces

24 h after the two phases were brought into contact in the hydrophilic capillary (see Fig.
5), we injected the aqueous solution (Figs. 9 a and b) so as to move the meniscus towards
the oil phase. During this injection, we use transmission microscopy to observe the evolution
of the (U) and (L) surfaces of the capillary on either side of the meniscus (see Figs. 9 c and
d). Additionally, we generate a 3D fluorescence reconstruction of the residual oil film that
forms after the meniscus displacement (Fig. 9 e).

Within this film, micro-droplets of water—initially formed on the capillary surface on
the oil side—become trapped (Figs. 9 c, d, and e). When these trapped droplets merge with the continuous aqueous phase, they rupture the thin interstitial oil film separating them, creating water patches in the residual film (Fig. 9 e). In what follows, we will refer to these water patches as holes. The thickness of the residual film is not uniform, being very thin near the triple line and gradually increasing towards the meniscus. Transmission images reveal that on surface (U) (Fig. 9 c), the droplets trapped in the residual film after the meniscus has passed are fewer in number but larger than those observed in the residual film on surface (L) (Fig. 9 d). After the meniscus passes, the trapped droplets continue to grow, while new droplets appear simultaneously in the film, as clearly shown in the images taken at *t*=3 *h* and *t*=6 *h* on surface (L) (Fig. 9 d).

After several tens of hours of injection, we stopped the injection and followed the evolution of the residual oil film left behind the meniscus at a position far from it, as shown in Fig. 10 a. Some of the droplets trapped within the film coalesce with the free water, forming numerous non-circular water patches (holes) in the residual oil film (see Fig. 10 b and c). These patches increase in size over time and when they come into contact with each other, they coalesce as shown in Figs. 10 b and form a cellular structure resembling that of a 2D foam, the continuous phase of which is oil containing fine droplets of water. The surface area of the capillary wall in contact with water increases over time as evident in Fig. 10 e.

## 3.2 b Hydrophobic Solid Surfaces

After 30 hours of contact between the two phases within the hydrophobic capillary—during which we monitored the auto-emulsification process (see Figs. 6 and 7)—the aqueous phase is injected. As the injection progresses, the meniscus advances toward the oily phase, leaving behind a residual oil film along the capillary surface (Fig. 11). The thickness of this film, estimated from vertical sections obtained by stacking XYZt fluorescence images (Fig. 11 d), measures a few micrometers.

Observations show that droplets entrapped in this oil film farther from the meniscus
<figure><img src="image_13.png" /><figcaption>Figure 9: a) Top view of the hydrophilic capillary showing the position of the field of observation relative to the triple line (represented by a dashed green line) and the meniscus (represented by a solid yellow line) at time t (the time origin is taken at the start of the injection phase). The middle of our field of view, which remains fixed during the injection phase, is at a distance $Δx_1=350$ µm from the initial position of the triple line. During the injection phase, the meniscus moves and crosses the field of observation. After six hours of injection, it is at a distance $Δx_2=250$ µm from the middle of our field of view. b) Diagram showing in the (XZ) plane the position of our fields of view on the (U) and (L) surfaces of the capillary and the displacement of the meniscus during the injection. c) and d) Transmission images of the (U) and (L) surfaces respectively. In these images, sessile water droplets can be seen to the left of the meniscus. To the right of the meniscus, the water droplets are trapped in the residual oil film that has formed on the (U) and (L) surfaces as a result of the meniscus displacement. e) 3D reconstruction showing the displacement of the meniscus towards the oil, the formation of the resulting residual oil film on the L surfaces of the capillary and the presence of water droplets trapped in this film.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Figure 10: a) Top view of the capillary, illustrating the field of view (black box) in relation to the initial triple line (green dotted line) and the meniscus (yellow solid line) after the injection has stopped in a hydrophilic capillary. The time origin corresponds to the cessation of injection. In this experiment, we have $\Delta x_1=380$ µm and $\Delta x_2=4$ mm. Images were acquired in transmission mode (b) and fluorescence mode (c), showing surface L at different times t. In the transmission images (b), orange dotted circles highlight the coalescence phenomenon between the holes. To the right of each fluorescence image (c), a corresponding view of the vertical section in the ($X=X_C$) plane of the residual oil film is provided. (d) Vertical sections of the residual oil film in the ($Y=Y_C$) plane at the same time t. (e) Perspective views of the residual films, reconstructed from 3D stacks of XYZ images.</figcaption></figure>
are larger than those closer to it. This phenomenon can be explained by the fact that the droplets initially present in the oil film—separating the meniscus from the capillary's solid surface—formed more rapidly before water injection than those on the free oil side of the meniscus. This is due to their proximity to the aqueous phase at the start of the injection, which enabled a faster initial supply of water molecules. Since osmotic pumping enhances diffusion over short distances, the closest droplets underwent a more rapid formation process. When the meniscus moves, the water droplets left behind in the oil film expand and merge with the free water, forming water patches within the residual oil film. These patches continue to grow over time, as shown in Figs. 11 c and d.

After the water injection stops, we examine the behavior of the (L) and (U) surfaces of the capillary near the new meniscus position. On the oil side, we observe the emergence of small isolated water droplets that nucleate and grow on the capillary walls, reaching an apparent diameter of approximately ten microns after 24 hours, with little variation beyond that point (Fig. 12). On the water side, within the oil film near the meniscus, we detect a dense phase of water droplets with a relatively narrow size distribution as shown in Fig. 13. The average droplet size gradually increases due to coalescence between the droplets.

## 3.3 Change in Solid Surface Wettability Before and After Water Injection

During the self-emulsification process and the injection of low salt water, the formation of microdroplets can potentially alter the wettability of walls. We herein analyze changes in the contact angle and the position of the triple line over time. Using stacks of XYZt fluorescence images, we reconstruct the 3D shape of the meniscus, allowing us to track the temporal evolution of the contact angle through vertical cross-sections taken at different time points. These cross-sections provide a precise and straightforward method for monitoring the position of the triple line and verifying the initial wettability of the capillary surface.
<figure><img src="image_15.png" /><figcaption>Figure 11: a) Top view of the hydrophobic capillary, illustrating the microscopic observation field (black boxes) relative to the triple line (green dotted line) and the meniscus (yellow solid line) during water injection. b) Transmission images of capillary surface I at different times t (the origin of time is taken when injection starts), with the advancing meniscus marked by a yellow solid line. c) Fluorescence images of the regions highlighted by blue boxes in a), captured at the same t times. Orange dotted lines indicate the positions X=X_c and Y=Y_c, corresponding to the vertical sections reconstructed in 3D from XYZ image stacks acquired at the same t instants. These sections are displayed to the left of each image in b) and c).</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure 12: a) Top view of the hydrophobic capillary, illustrating the position of our microscopic observation fields (black boxes) relative to the initial triple line (green dotted line) and the meniscus (yellow solid line) after end of injection. The distances in these experiments are $\Delta x_1=4.3$ mm and $\Delta x_2=380$ µm. b) Schematic vertical cross-section of the capillary, showing the positions of the meniscus and our observation field. c) Transmission images of surface U of the capillary, corresponding to the red box in figure b), captured at different time points (time origin is the end of the injection).</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 13: a) Top view of the hydrophobic capillary, showing the positions of our microscopic observation fields (black boxes) relative to the triple line (green dotted line) and the meniscus (yellow solid line) after end of injection. The distances in these experiments are $\Delta x_1=600 \ \mu m$ and $\Delta x_2=4 \ mm$. b) Schematic vertical cross-section of the capillary, illustrating the locations of the meniscus and our observation field. c) Transmission images of surface L of the capillary, corresponding to the blue box in figure b), captured at different time points.</figcaption></figure>
### 3.3 a Hydrophilic Solid Surface

In all our experiments, a slight displacement of the triple line occurs after the two phases come into contact during the rest period, with varying degrees of intensity. In some cases, this displacement may result from capillary filling issues, where small bubbles become trapped in the corners. It can also be caused by the presence of a dead volume, which shifts the triple line when the valves are opened. However, even when these factors are controlled, the triple line displacement still occurs, suggesting that this phenomenon is naturally linked to the self-emulsification process.

Fig. 14 shows how the shape of the meniscus evolves before and during low salt water injection. On hydrophilic surfaces, W/O microdroplets spontaneously form near the triple line on capillary structures. Over time, these droplets grow in size and multiply, eventually merging upon contact as detailed in the previous sections. Their presence induces a notable shift in the contact angle and alters the meniscus position as shown in Figs. 14 b. Initially, the contact angle on a hydrophilic surface is low (typically ≈ 20°), reflecting strong water wettability. However, as microdroplets develop and expand, the apparent contact angle progressively increases as reported on Figs. 14 a and b.

The non-uniform movement of the meniscus is strongly influenced by the formation of sessile micro-droplets. When they appear, these droplets can immobilise the triple line, leading to a *stick-slip* type phenomenon observed for low water droplet densities. This mechanism, already well documented in the literature for droplet evaporation,²⁴,²⁵ is usually characterised by alternating phases of stick and slip of the contact line on the surface. During the injection process, we observe that the value of the contact angle, Θ rapidly increases to reach 180° while the meniscus advances, leaving behind a residual oil film containing trapped droplets. These droplets eventually coalesce with the continuous phase, forming water patches in the residual oil film (Fig. 14), while the triple line slightly shifts towards the meniscus.
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>Figure 14: a) Sequence of 3D fluorescence images used to reconstruct the meniscus shape, along with images captured on the lower surface L of the hydrophilic capillary at different time points: after the two fluids are brought into contact, before LWS injection and during the injection of the low salinity water solution. b) Kinetic evolution of the apparent contact angle and the movement of the triple line before and during LWS injection. c) Vertical sections in the plane (XZ) illustrating the evolution of the shape of the meniscus before and during LWS injection.</figcaption></figure>
### 3.3 b Hydrophobic Solid Surface

As with the hydrophilic capillary, we monitor the evolution of wettability immediately after filling both phases, using confocal microscopy in fluorescence mode and 2D analysis at an observation point where the triple line is visible (Fig. 15). The value of the contact angle measured at the moment the two phases come into contact ($\Theta \simeq 160^{\circ}$) clearly confirms the hydrophobic nature of the capillary walls.

Observations are first conducted before the injection of the low salt water, allowing us to investigate how the spontaneous formation of microdroplets—primarily occurring at the macroscopic water/oil interface, near the meniscus, due to the hydrophobicity of the capillary walls affects surface wettability. The dynamics of their formation and growth differ from those on hydrophilic surfaces. Droplets on hydrophobic surfaces tend to be larger and have a greater overall volume, yet they are fewer in number on the surface. In addition, because of the hydrophobicity of the walls, numerous microdroplets rapidly form along the edge of the meniscus, and some eventually create, as they merge with free water, some water patches within the thin oil film separating the meniscus from the solid surface. The thinner the oil film—especially closer to the triple line—the greater the number of droplets that form over time. Before injection, we observe no significant changes in the shape of the meniscus over time, in contrast to what was observed in hydrophilic capillaries.

After 20 hours of contact, we inject 2 $\mu$L of low salt water over a 6-hour period, during which we followed the displacement and shape of the meniscus. For this study, we select an observation point located away from the meniscus's initial position to examine its behavior as it approaches (Fig. 16 d). The reference time $t=0$ is set at the moment the injection stops. On surface (L), only a few microdroplets, each smaller than 5 $\mu$m, are visible, spaced far apart from one another. As shown in Fig. 16, during the injection phase, a residual oil film remains on the surface along the entire path of the meniscus. Some trapped sessile microdroplets immediately form holes, while others continue to grow. After the injection, the contact angle remains close to 180° because of the residual oil film left on the capillary surface as the
meniscus advances. Although this oil film gradually evolves-forming an increasing number
of water patches through interaction with the continuous aqueous phase- the surface coverage
of these patches remains very limited near the meniscus. This limited coverage explains why
the contact angle remains close to 180°.

<figure><img src="image_21.png" /><figcaption>Figure 15: a) and b) 3D fluorescence visualizations from two different angles of the meniscus before the LWS injection in the hydrophobic capillary (time is measured from the moment the two fluids first come into contact within the capillary). c) 2D fluorescence profile of the X-Z cross-section of the meniscus at the same time points. d) Top view of the capillary indicating the field of the microscopic observations and its positions relative to the initial triple line (dashed green line) and meniscus (yellow line).</figcaption></figure>

4. Conclusion

Our experimental study on wettability effects during low-salinity water flooding provides
valuable insights into the dynamic behavior of the contact angle, triple line movement, and
<figure><img src="image_22.png" /><figcaption>Figure 16: a) Diagram illustrating the advancement of the meniscus during LWS injection; b) 3D fluorescence visualizations of the meniscus during the LWS injection in the hydrophobic capillary (time is measured from the moment the LWS injection starts). c) 2D fluorescence profile of the X-Z cross-section of the meniscus at the same time points. d) Top view of the capillary indicating the field of the microscopic observations and its positions relative to the initial triple line (dashed green line) and meniscus (yellow line).</figcaption></figure>
meniscus evolution on both hydrophilic and hydrophobic capillary surfaces. Our observations confirm that pristine wettability plays a crucial role in the self-emulsification process and subsequent oil mobilization.

On hydrophilic surfaces, W/O microdroplets spontaneously form on the wall surface near the triple line, leading to a progressive increase in the apparent contact angle. These droplets alter the movement of the meniscus, causing a stick-slip phenomenon that influences oil displacement.

Conversely, on hydrophobic surfaces, W/O microdroplets tend to be larger but fewer in number. They primarily form at the macroscopic oil-water interface and along the meniscus edge, creating water domains that affect the stability of the residual oil film. Despite prolonged contact, the contact angle remains close to 180°, indicating persistant oil adhesion. During injection, the meniscus moves forward, leaving behind an oil film containing trapped W/O microdroplets. These micro-droplets play a crucial role in fluid transport and recovery efficiency, as they merge with the free water, creating water patches within the residual oil film left on the solid surface by the passage of the meniscus. The surface fraction occupied by these water domains increases over time until the wall surface becomes completely wetted by water.

Overall, this study underscores the importance of wettability in controlling W/O microdroplet spontaneous formation and oil displacement mechanisms. Understanding these interactions provides critical insights for optimizing enhanced oil recovery strategies through controlled salinity water flooding.

## Acknowledgement

This work was funded by a French CIFRE fellowship (contract number 2021-052) financed by TotalEnergies S.E. We acknowledge TotalEnergies S.E. management for permission to publish this work. We thank N. Passade-Boupat for fruitful discussions.
References

(1) Lake, L. W. *Enhanced Oil Recovery*; Prentice Hall: Englewood Cliffs, 1989.

(2) Alvarado, V.; Manrique, E. Enhanced Oil Recovery: An Update Review. *Energies* **2019**, *3*, 1529–1575.

(3) Bernard, G. G. Effect of Floodwater Salinity on Recovery of Oil from Cores Containing Clays, SPE California Regional Meeting, Society of Petroleum Engineers, 1967.

(4) Tang, G. Q.; Morrow, N. R. Salinity, Temperature, Oil Composition, and Oil Recovery by Waterflooding. *SPE Reservoir Engineering* **1997**, *12*, 269–276.

(5) Yildiz, H. O.; Morrow, N. R. Effect of brine composition on recovery of Moutray crude oil by waterflooding. *Journal of Petroleum Science and Engineering* **1996**, *14*, 159–168.

(6) Chavan, M.; Dandekar, A.; Patil, S.; Khataniar, S. Low-salinity-based enhanced oil recovery literature review and associated screening criteria. *Petroleum Science* **2019**, *16*, 1344–1360.

(7) Drummond, C.; Israelachvili, J. Fundamental studies of crude oil-surface water interactions and its relationship to reservoir wettability. *Journal of Petroleum Science and Engineering* **2004**, *45*, 61–81.

(8) Yousef, A. A.; Al-Saleh, S.; Al-Kaabi, A.; Al-Jawfi, M. Laboratory Investigation of Novel Oil Recovery Method for Carbonate Reservoirs. All Days SPE-137634-MS. Calgary, Alberta, Canada, 2010.

(9) Song, J.; Rezaee, S.; Guo, W.; Hernandez, B.; Puerto, M.; Vargas, F.; Hirasaki, G.; Biswal, S. Evaluating physicochemical properties of crude oil as indicators of low-salinity-induced wettability alteration in carbonate minerals. *Scientific Reports* **2020**, *10*, 3762.
(10) Sheng, J. Critical review of low-salinity waterflooding. *Journal of Petroleum Science and Engineering* **2014**, *120*, 216-224.

(11) AlHammadi, M.; Mahzariand, P.; Sohrabi, M. Fundamental investigation of underlying mechanisms behind improved oil recovery by low salinity water injection in carbonate rocks. *Fuel* **2018**, *220*, 345-357.

(12) Emadi, A.; Sohrabi, M. Visual Investigation of Oil Recovery by Low Salinity Water Injection: Formation of Water Micro-Dispersions and Wettability Alteration. 2013.

(13) Mahzari, P.; Sohrabi, M.; Façanha, J. The Decisive Role of Microdispersion Formation in Improved Oil Recovery by Low-Salinity-Water Injection in Sandstone Formations. *SPE Journal* **2019**, *24*, 2859-2873.

(14) Duboué, J.; Bourrel, M.; Santanach-Carreras, E.; Klimenko, A.; Agenet, N.; Passade-Boupat, N.; Lequeux, F. Auto-Emulsification of Water at the Crude Oil/Water Interface: A Mechanism Driven by Osmotic Gradient. *Energy Fuels* **2019**, *33*, 7020-7027.

(15) Crestel, E.; Kvasnickova, A.; Santanach-Carreras, E.; Bibette, E. Motion of oil in water induced by osmosis in a confined system. *Physical Review Fluids* **2020**, *5*, 104003.

(16) Sandengen, K.; Kristoffersen, A.; Melhuus, K.; Jøsang, L. Osmosis as Mechanism for Low-Salinity Enhanced Oil Recovery. *SPE Journal* **2016**, *21*, 1227-1235.

(17) Fredriksen, S.; Rognmo, A.; Sandengen, K.; Fernø, M. Wettability Effects on Osmosis as an Oil-Mobilization Mechanism During Low-Salinity Water Flooding. *Petrophysics* **2016**, *58*, 28-35.

(18) Fredriksen, S.; Rognmo, A.; Fernø, M. Pore-scale mechanisms during low salinity waterflooding: Oil mobilization by diffusion and osmosis. *Journal of Petroleum Science and Engineering* **2018**, *163*, 650-660.
(19) Selem, A. M.; Agenet, N.; Blunt, M. J.; Bijeljic, B. Pore-scale processes in tertiary low salinity waterflooding in a carbonate rock: Micro-dispersions, water film growth, and wettability change. *J. Colloid Interface Sci.* **2022**, *628*, 486–498.

(20) Selem, A. M.; Agenet, N.; Foroughi, S.; Blunt, M. J.; Bijeljic, B. Pore-Scale Imaging of Emulsification of Oil during Tertiary and Secondary Low Salinity Waterflooding in a Reservoir Carbonate. *Energy Fuels* **2023**, *37*, 1636816377.

(21) Goncalves, S.; Castillo, J.; Fernández, A.; Hung, J. Absorbance and Fluorescence Spectroscopy on the Aggregation Behavior of Asphaltene–Toluene Solutions. *Fuel* **2004**, *83*(13), 1823–1828.

(22) Brosh, E.; Kiv, A. Radiogenic silson precipitates in neutron irradiated aluminium. *J. Nucl. Mater.* **2002**, *306*, 173–179.

(23) Nozawa, K.; Delville, M.; ushiki, H.; Panizza, P.; Delville, J. Growth of monodisperse mescopic metal-oxide colloids under constant monomer supply. *Phys. Rev. E* **2005**, *72*, 011404.

(24) Shanahan, M. E. Simple Theory of Stick-Slip Wetting Hysteresis. *Langmuir* **1995**, *11*, 1041–1043.

(25) Kim, D.-O.; Pack, M.; Rodoni, A.; Kaneelil, P.; Sun, Y. The Effect of Particle Wettability on the Stick-Slip Motion of the Contact Line. *Soft Matter* **2018**, *14*, 9599–9608.
<figure><img src="image_23.png" /><figcaption>TOC graphic</figcaption></figure>