# Wetting of Silicone Oil Drops and Air Bubbles in Ethylene Glycol/Water Mixtures

Philippe Kunemann, Mohammed Djamel Bendimerad, Thibault Ngadande

Nerum, Olga Voytko, Manon Guyot, Laurent Vonna

► To cite this version:

Philippe Kunemann, Mohammed Djamel Bendimerad, Thibault Ngadande Nerum, Olga Voytko, Manon Guyot, et al.. Wetting of Silicone Oil Drops and Air Bubbles in Ethylene Glycol/Water Mixtures. Langmuir, 2025, 10.1021/acs.langmuir.4c03817 . hal-04973410

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# Wetting of Silicone Oil Drops and Air Bubbles in Ethylene Glycol/Water Mixtures

Philippe Kunemann, Mohammed Djamel Bendimerad, Thibault Ngadande Nerum, Olga Voytko, Manon Guyot, and Laurent Vonna*

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Cite This: https://doi.org/10.1021/acs.langmuir.4c03817

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## ACCESS

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Supporting Information

**ABSTRACT:** Contact angles of silicone oil droplets and air bubbles settled on a solid substrate and confined in binary ethylene glycol/water mixtures were studied as a function of ethylene glycol concentration. Despite similar reductions in interfacial tensions for both systems, distinct wetting behaviors were observed. While the air bubble contact angle increased with ethylene glycol concentration, the silicone oil droplet exhibited a more complex response, characterized by a stable contact angle at low ethylene glycol concentrations followed by an increase at higher concentrations. This suggests that droplet morphology can remain relatively stable even as interfacial tensions change. A phenomenological description based on the imbalanced forces highlights the crucial role of the contact angle in pure water environment, in predicting the wetting behavior that may follow the reduction of interfacial tension by further surfactant or cosolvent adsorption.

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## INTRODUCTION

Coupled interfaces, which are ubiquitous in natural and engineered systems, arise at the junction of three immiscible phases. When one of these phases is a liquid containing surfactants or cosolvents, their adsorption at interfaces can significantly impact interfacial tension, potentially triggering a rearrangement at the triple line. Droplets or gas bubbles resting on a solid substrate and immersed in another immiscible liquid are common examples of such coupled interfaces. The wetting behavior of such confined droplets and bubbles when interfacial tensions are reduced by adsorbed species, plays a critical role in numerous technologies, including microfluidics¹⁻³ and oil removal.⁴⁻⁶

The Young equation, which accounts for changes in surface tension due to adsorption, accurately describes the final equilibrium shape of confined droplets or bubbles.⁷ However, a complete description frequently necessitates consideration of the interplay between wetting, frictional, and viscous forces.⁵,⁸,⁹ Beyond simple spreading or shrinking, complex phenomena such as interfacial oscillations or Marangoni flows can emerge,¹⁰⁻¹⁵ including transient equilibrium states as shown in our previous work.¹⁶,¹⁷

This study investigates the impact of ethylene glycol (EG) on the wetting behavior of air bubbles and silicone oil droplets immersed in binary EG-water (EG-W) mixtures. These mixtures are widely used across diverse fields because of their tunable properties.¹⁸ Beyond their well-known bulk properties such as viscosity and dielectric constant the interfacial behavior of EG-W mixtures plays a crucial role in numerous applications. For instance, the molecular-scale structure of

the mixture and its organization around biological objects is known to significantly affect enzyme and protein dynamics.¹⁹⁻²¹ In materials science, EG-W mixtures have been proposed for the synthesis of mesoporous materials²² or the elaboration of nanofluids with enhanced thermal properties.²³ Similarly, in microfluidics, EG-W mixtures are frequently employed to control the viscosity of the liquid environment, impacting microdroplet formation, transport, and adhesion at T-junctions.²⁴,²⁵

Despite the prevalence of EG-W mixtures in various fields, a knowledge gap persists regarding the impact of cosolvent adsorption, such as that of EG, on the interfacial properties and associated wetting and adhesive phenomena in these binary mixtures. Although less potent than surfactants, EG can also reduce interfacial tension.²⁶,²⁷ To address this gap, this study investigates the influence of EG adsorption on wetting behavior in confined geometries. Specifically, we examined the reduction of interfacial tension by EG adsorption on coupled interfaces through the shape analysis of settled oil droplets and air bubbles exposed to varying EG concentrations. Our approach emphasizes the competitive interplay of interfacial tension changes due to cosolvent adsorption in predicting the contact angle variations. We demonstrate that

Received: September 27, 2024

Revised: February 2, 2025

Accepted: February 18, 2025
these contact angles cannot be solely predicted by independent surface tension variations with EG concentration, and that the contact angle of the oil droplet or air bubble significantly influences subsequent changes.

## ■ MATERIALS AND METHODS

**Experimental Setup and Procedure.** The experimental setup consisted of a glass cell filled with a binary ethylene glycol-water (EG-W) mixture. A hydrophobic substrate was immersed in the mixture, upon which a 2 µL silicone oil droplet or air bubble was deposited. Silicone oil with a low-viscosity (10 mPa·s), polydimethylsiloxane (PDMS) from abcr GmbH (Karlsruhe, Germany), exhibited a surface tension of 20 mN/m. Fresh binary EG-W mixtures were prepared using analytical-grade ethylene glycol (EG) from Merck KGaA (Darmstadt, Germany) and deionized water (W). The EG concentrations are expressed in the following as molar fraction. The measured surface tension was 48 mN/m for neat ethylene glycol (viscosity, 20 mPa·s) and 73 mN/m for water.

**Contact Angle Measurements.** The contact angles of the silicone oil droplets and air bubbles on a hydrophobic substrate immersed in binary ethylene glycol-water (EG-W) mixtures were determined as a function of ethylene glycol concentration using a DSA100 Drop Shape Analyzer from Krüss Scientific (Hamburg, Germany) equipped with a video camera. Measurements were conducted under ambient laboratory conditions (approximately 20 °C and 35% RH) for EG concentration (molar fraction) $x_{EG}$ ranging from 0 to 1. The measurements were repeated four to ten times in independent experiments, using new samples (see Supporting Information for the complete data set). Contact angles were recorded after 1 h of exposure, during which no significant changes were observed for both PDMS droplets and air bubbles. To ensure stability, additional measurements for PDMS droplets were performed over a period of 12 h, with precautions taken to prevent water evaporation. The stability of the contact angle during this extended period suggests that the interface reaches equilibrium and remains unchanged within the time scale of our experiments. At low ethylene glycol (EG) concentrations, the contact angles remained stable. However, at a molar fraction of 0.7 EG in water, a slow decrease of 4–5° was observed, which does not significantly affect our conclusions. This observation supports the assumption that EG diffusion into the PDMS phase is either negligible or too slow to significantly alter the interfacial properties during our measurements. For air bubbles, prolonged measurements were not feasible due to condensation at the triple line, causing abrupt changes in the contact angle.

**Hydrophobic Substrate.** A hydrophobic substrate was prepared by grafting hexadecyltrichlorosilane (HTS) from abcr GmbH (Karlsruhe, Germany) onto a native silicon oxide layer. Silicon wafers (2 cm × 2 cm) were cleaned in a cyclohexane ultrasonic bath and dried under nitrogen flow. Subsequently, they were immersed in piranha solution (30% H₂O₂/H₂SO₄, 3:7 v/v) at 60 °C for 30 min, followed by rinsing with deionized water and drying under nitrogen flow. Finally, the wafers were immersed in a 10⁻³ mol/L cyclohexane solution of HTS for diff h, yielding a self-assembled monolayer with hydrophobic properties. Characterization of the HTS layer confirmed that the resulting coating, while uniform on a macroscopic scale, does not form a perfectly dense and ordered monolayer. Ellipsometric measurements revealed an average thickness of 9 Å, significantly lower than the 24 Å expected for a fully extended monolayer, suggesting a structure where alkyl chains are predominantly lying on the surface rather than standing upright. AFM measurements over an area of 100 µm² yielded a root-mean-square (RMS) roughness of 0.28 nm, confirming a low topographical roughness. Contact angle measurements revealed an advancing contact angle of 114° and a receding contact angle of 105°, resulting in a hysteresis of 9°. The static contact angle was measured at 112°, indicating a smooth macroscopic wetting behavior despite potential molecular-scale heterogeneities. These results suggest that while the HTS layer effectively screens underlying polar sites such as unreacted silanol groups, subtle deviations from an ideal monolayer must be considered when interpreting the wettability

of the surface. The presence of lying alkyl chains and possible silanol patches may contribute to a slightly higher actual surface energy than the commonly cited 21 mJ/m² for methyl-terminated SAMs.²⁸,²⁹

**Interfacial Tensions of EG-W Mixtures against Air, PDMS and HTS.** Interfacial tensions between EG-W mixtures and both air and PDMS, $γ_{air/M}(x_{EG})$ and $γ_{PDMS/M}(x_{EG})$ respectively, were measured as a function of EG concentration using a K100 tensiometer from Krüss Scientific (Hamburg, Germany) and the Wilhelmy plate method. A platinum plate, rinsed with ethanol, and flamed served as a Wilhelmy plate. For the PDMS-mixture interfacial tension measurements, the platinum plate was first immersed in the EG-W mixture, followed by the addition of PDMS oil to the top of the mixture. The measurements were repeated three times with newly flamed platinum plates (see Supporting Information for the complete data set).

The interfacial tension between the mixture and the hydrophobic substrate $γ_{HTS/M}(x_{EG})$ was determined as a function of EG concentration using the following Young equation using the contact angle of droplets of EG-W mixtures settled on the substrate in air

$$γ_{HTS/M}(x_{EG}) = γ_{HTS} - γ_{air/M}(x_{EG})\cos θ_{e}(x_{EG}) \quad (1)$$

In this equation, the surface energy of the hydrophobic substrate $γ_{HTS}$ was set to 21 mJ/m² as measured elsewhere,²⁸ the interfacial tension between air and EG-W mixture $γ_{air/M}(x_{EG})$ was obtained from tensiometric measurements as described previously, and the contact angle $θ_{e}(x_{EG})$ was obtained from contact angle measurements. Contact angles were measured at least on two different droplets for two different samples (see Supporting Information for the complete data set).

The three interfacial tensions $γ_{air/M}(x_{EG})$, $γ_{PDMS/M}(x_{EG})$ and $γ_{HTS/M}(x_{EG})$, measured as described previously, are shown in Figure 1.

<figure><img src="image_8.png" /><figcaption>**Figure 1.** Interfacial tension of the EG-W mixtures against air, $γ_{air/M}$(○), against PDMS, $γ_{PDMS/M}$(red □), and against hydrophobic substrate HTS, $γ_{HTS/M}$(Δ), as a function of the EG molar fraction. The solid lines correspond to the fit of the experimental data using the equation of Connors and Wright.¹⁷</figcaption></figure>

1 as a function of the EG concentration. As expected, the interfacial tension decreases with increasing EG concentration in the mixture for all three interfaces. Our experimental interfacial tension values for the EG-W mixtures and both air and PDMS closely fit those previously reported.²⁴,³⁰

## ■ THEORETICAL BACKGROUND

A force balance model was developed at the triple line to predict the wetting behavior of droplets and bubbles. This model considers the variation of interfacial tensions with EG
<figure><img src="image_9.png" /><figcaption>**Figure 2.** Schemes illustrating the three possible force imbalances at the triple line, leading to (a) a spreading of the drop (decrease in $\theta_e^0$), (b) a shrinking of the droplet (increase in $\theta_e^0$) and (c) no changes ($\theta_e^0$ constant). The interfacial tensions decrease upon ethylene glycol adsorption (from ① to ②) whereas the contact angle of the system is virtually constrained to retain its value, $\theta_e^0$. The relative variations in the interfacial tension between the substrate and the mixture, $|d\gamma_{HTS/M}|$, and the in-plane projected interfacial tension between the drop and the mixture, $|d\gamma_{drop/M}|\cos\theta_e^0$, are sketched at the bottom of each figure for comparison.</figcaption></figure>

concentration relative to a pure water reference state as surrounding medium. The resulting force balance allows for the prediction of contact angle changes and droplet or bubble reconfiguration. In the following, “HTS” refers to the hydrophobic substrate, and “drop” denotes either the oil droplet or air bubble, for which the same theoretical framework applies.

In the following we consider that modifications in interfacial tensions due to variation in EG concentration primarily concern the interfaces involving ethylene glycol-water (EG-W) mixtures. Owing to the low miscibility of EG in PDMS, the interfacial tension between the PDMS droplet and the substrate remains largely unaffected by the EG concentration. Similarly, the gas-phase nature of the air bubble prevents significant adsorption of EG at the air bubble/substrate interface, resulting in a constant interfacial tension for this system. The interfacial tension between the hydrophobic substrate and PDMS, and between the hydrophobic substrate and air, written $\gamma_{drop/HTS}^0$ in the following discussion, will thus be considered constant.

The equilibrium state of the system for a pure water (W) reference state as the surrounding medium is described by Young’s equation

$$ \gamma_{HTS/W}^0 - \gamma_{drop/HTS}^0 = \gamma_{drop/W}^0 \cos \theta_e^0 \quad (2) $$

where $\gamma_{HTS/W}^0$, $\gamma_{drop/HTS}^0$, $\gamma_{drop/W}^0$ and $\theta_e^0$ represent, the interfacial tensions and equilibrium contact angle. The exponent “0” in this equation refers to the pure water reference state. For a system consisting of a PDMS droplet or air bubble in equilibrium with a binary EG-W mixture (M) of concentration $x_{EG}$, Young’s equation can be written as

$$ \gamma_{HTS/M} - \gamma_{drop/HTS} = \gamma_{drop/M} \cos \theta_e(x_{EG}) \quad (3) $$

where “M” refers to the EG-W mixture of concentration $x_{EG}$.

The variation in the interfacial tension relative to the pure water reference state is defined as $d\gamma = \gamma(x_{EG}) - \gamma^0$, or $\gamma(x_{EG}) = d\gamma + \gamma^0$. Considering that $d\gamma$ is negative due to EG adsorption, this yields the relationship $\gamma(x_{EG}) = \gamma^0 - |d\gamma|$. This relationship can now be applied to the interfacial tensions involving the EG-W mixture, $\gamma_{HTS/M}$ and $\gamma_{drop/M}$ in eq 3, leading to the following equation

$$ \begin{aligned} & (\gamma_{HTS/W}^0 - |d\gamma_{HTS/M}|) - \gamma_{drop/HTS}^0 \\ &= (\gamma_{drop/W}^0 - |d\gamma_{drop/M}|)\cos \theta_e(x_{EG}) \end{aligned} \quad (4) $$

In this concentration-dependent equilibrium equation, the way $\theta_e(x_{EG})$ varies with respect to the virgin state $\theta_e^0$ is a priori unknown but strictly depends on the balance between $|d\gamma_{HTS/M}|$ and $|d\gamma_{drop/M}|\cos\theta_e^0$. Thus, we can predict whether the drop will shrink or spread with respect to $\theta_e^0$ by considering the following three hypothetical cases where we suppose that the interfacial tensions have changed upon EG adsorption whereas the contact angle of the system is virtually constrained to retain its value, $\theta_e^0$. This simply consists of rewriting eq 4 with $\theta_e(x_{EG}) = \theta_e^0$, and looking for the sign of the resulting force imbalance.

**Case 1.** The adsorption and interfacial tension changes are such that $|d\gamma_{HTS/M}| < |d\gamma_{drop/M}|\cos\theta_e^0$ (Figure 2a). The balance of forces given by eq 3 is virtually modified to

$$ \begin{aligned} & (\gamma_{HTS/W}^0 - |d\gamma_{HTS/M}|) - \gamma_{drop/HTS}^0 \\ & > (\gamma_{drop/W}^0 - |d\gamma_{drop/M}|)\cos\theta_e^0 \end{aligned} \quad (5) $$

It is then easy to see that if the system was free to evolve (release of constraint on $\theta$), the drop would have spread, driven by a higher left-hand surface force. In other words, the system should always evolve in this case toward new equilibrium states of lower $\theta$ with respect to $\theta_e^0$, or higher cosine, as defined by

$$ \begin{aligned} & (\gamma_{HTS/W}^0 - |d\gamma_{HTS/M}|) - \gamma_{drop/HTS}^0 \\ &= (\gamma_{drop/W}^0 - |d\gamma_{drop/M}|)\cos(\theta_e^0 - \delta\theta) \end{aligned} \quad (6) $$

This corresponds to the spreading of the droplet or bubble.

**Case 2.** The adsorption and interfacial tension changes are such that $|d\gamma_{HTS/M}| > |d\gamma_{drop/M}|\cos\theta_e^0$ (Figure 2b). This situation is the symmetrical force balance given by eq 4

$$ \begin{aligned} & (\gamma_{HTS/W}^0 - |d\gamma_{HTS/M}|) - \gamma_{drop/HTS}^0 \\ & < (\gamma_{drop/W}^0 - |d\gamma_{drop/M}|)\cos\theta_e^0 \end{aligned} \quad (7) $$

The free evolution of the system, driven by the higher right-hand side wetting force, requires $\cos\theta$ to decrease (increase in $\theta$) with respect to $\theta_e^0$. As a result, the drop shrinks toward the new equilibrium state given by
$$ (\gamma_{HTS/W}^0 - |d\gamma_{HTS/M}|) - \gamma_{drop/HTS}^0 = (\gamma_{drop/W}^0 - |d\gamma_{drop/M}|)\cos(\theta_e^0 + \delta\theta) \quad (8) $$

This corresponds to the shrinking of the droplet or bubble.

**Case 3.** The third case corresponds to the symmetric variation of both interfacial tension such that $|d\gamma_{HTS/M}| = |d\gamma_{drop/M}| \cos \theta_e^0$ (Figure 2c). In this case, no evolution of the system is observed despite the variation in interfacial tensions.

In the following, this simple formulation of the capillary force imbalance that results from the adsorption is used to analyze the observed wetting behaviors of the different systems in the binary mixtures.

## ■ RESULTS AND DISCUSSION

This section presents and discusses the experimental results on how air bubbles and silicon oil (PDMS) droplets behave in ethylene glycol-water (EG/W) mixtures, based on the theoretical framework established in the Section Theoretical Background. We first examined the effect of the EG concentration on the air bubble system, followed by a similar analysis of the silicon oil droplet system. Finally, we compare the wetting behavior observed in both systems and propose an explanation for the differences observed between the two systems.

**Air Bubble System.** Figure 3 shows the static contact angle measured for an air bubble settled on an HTS substrate

<figure><img src="image_10.png" /><figcaption>**Figure 3.** Contact angle of the air bubble as a function of EG molar fraction in the surrounding mixture. The inset corresponds to the dependence of the cosine of the contact angle with the EG molar fraction.</figcaption></figure>

in mixtures of ethylene glycol (EG) and water, plotted as a function of EG concentration in the mixture. The contact angle increased steadily from ~76° toward a maximum of ~101°, from the pure water environment ($x_{EG} = 0$) to the pure EG environment ($x_{EG} = 1$). We note that contact angles are conventionally defined from the denser phase (i.e., the liquid). Thus, the angles measured inside the air bubble correspond to 180°—CA.

As exposed in the Section Theoretical Background, the analysis of the imbalanced forces allows us to predict and explain the changes in contact angle with varying EG concentration for both bubbles and droplets. In the air bubble

system, this force arises from the interplay between variations in the HTS/mixture interfacial tension ($|d\gamma_{HTS/M}|$) and the in-plane projected air/mixture interfacial tension ($|d\gamma_{air/M}| \cos \theta_e^0$), both influenced by EG concentration. Figure 4 shows these

<figure><img src="image_11.png" /><figcaption>**Figure 4.** Variation in HTS/mixture interfacial tension $|d\gamma_{HTS/M}|$ and projected air/mixture interfacial tension ($|d\gamma_{air/M}| \cos \theta_e^0$) as a function of EG concentration in the mixture ($x_{EG}$).</figcaption></figure>

variations obtained from our experimental data (Figure 1) and a bubble contact angle of $\theta_e^0 = 76°$ for the pure water environment. The larger decrease in HTS/mixture interfacial tension $|d\gamma_{HTS/M}|$ compared to the in-plane projected air/mixture interfacial tension $|d\gamma_{air/M}| \cos \theta_e^0$ results in an imbalanced force that leads to bubble shrinkage, or equivalently, an increase in the contact angle (Figure 3). This wetting behavior corresponds to case 2 described in the Section Theoretical Background ($|d\gamma_{HTS/M}| > |d\gamma_{air/M}| \cos \theta_e^0$), leading to a monotonic rise in $\theta(x_{EG})$.

**Silicon Oil (PDMS) Droplet System.** Unlike the air bubble system (Figure 3), the silicon oil (PDMS) droplet exhibited a more complex dependence on EG concentration (Figure 5). The contact angle of the silicon oil droplet remained constant (~25°) and showed a slight decrease with increasing EG concentration from $x_{EG} = 0$ to $x_{EG} = 0.2$. It then increased to a maximum of approximately 48° with increasing EG concentrations.

Similar to the air bubble discussion, this behavior can be analyzed based on an imbalanced force as described in the Section Theoretical Background. In the silicon oil (PDMS) droplet system, this force arises from the interplay between the variations in the HTS/mixture interfacial tension $|d\gamma_{HTS/M}|$ and the in-plane projected PDMS/mixture interfacial tension $|d\gamma_{PDMS/M}| \cos \theta_e^0$, both influenced by the EG concentration. Figure 6 shows the variations obtained from our experimental data (Figure 1) and a silicon oil (PDMS) drop contact angle of $\theta_e^0 = 25°$ for the pure water environment. Contrary to the air bubble system (Figure 4), these variations are similar in magnitude, with $|d\gamma_{HTS/M}|$ slightly higher than $|d\gamma_{PDMS/M}| \cos \theta_e^0$. This situation where $|d\gamma_{HTS/M}| > |d\gamma_{PDMS/M}| \cos \theta_e^0$, explains the increase in the contact angle for EG concentration >0.2, according to case 2 exposed the Section Theoretical Background. However, it does not explain the constant contact angle measured for EG concentrations <0.2, for which similar variations in the interfacial tensions are expected, $|d\gamma_{HTS/M}| = |d\gamma_{air/M}| \cos \theta_e^0$.
<figure><img src="image_12.png" /><figcaption>**Figure 5.** Contact angle of silicon oil drop as a function of EG molar fraction in surrounding mixture. The inset corresponds to the dependence of the cosine of the contact angle with the EG molar fraction.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 6.** Variations in the HTS/mixture interfacial tension (|dγ<sub>HTS/M</sub>|) and projected bubble/mixture interfacial tension (|dγ<sub>PDMS/M</sub>| cos θ<sub>e</sub><sup>0</sup>) as a function of EG concentration in the mixture (x<sub>EG</sub>).</figcaption></figure>

dγ<sub>drop/M</sub> cos θ<sub>e</sub><sup>0</sup>, according to case 3 exposed the Section Theoretical Background.

The observed discrepancy between the theoretical model and experimental data for the silicon oil (PDMS) droplet system underscores the limitations of solely considering interfacial tension variations, particularly in the context of the silicon oil droplet. The interfacial tension variations used in the theoretical model were derived from independent measurements using Wilhelmy's method (see Section Materials and Methods) and, in the particular case of the silicon oil drop system, are quite close to each other. These results strongly suggest that the interplay between the coupled interfacial tensions in the configuration of the silicon oil droplet plays a crucial role which cannot be anticipated when the interfacial tension variations are close to each other and determined independently. This is less evident in the case of the air bubble system, for which the variations in the interfacial tensions are

by far more distinct, showing a clear correlation with the
theoretical prediction (Figure 4).

We would like to emphasize that the interpretation presented in this study is based on the strict application of Young's equation, which assumes equilibrium conditions. However, the experimental contact angle we discuss corresponds to the observed static contact angle, not the true equilibrium contact angle. To minimize potential discrepancies between the static and equilibrium contact angles, we have used HTS self-assembled monolayers adsorbed onto extremely smooth silicon substrates (see Section Materials and Methods). These substrates were chosen to reduce anchoring effects of the triple line caused by topographical or chemical defects, aiming to approximate equilibrium conditions as closely as possible with the measured static contact angle. It is worth noting that for water droplets on these surfaces, the static contact angle differs only slightly from the advancing contact angle, with a difference of approximately 2°, indicating a smooth and homogeneous surface with minimal defects. In addition to the potential difference between the equilibrium contact angle (used in our theoretical model) and the measured static contact angle, we also acknowledge that droplets and bubbles may deform under buoyancy forces. Nevertheless, in the specific case of air bubbles, our model accurately predicts the observed evolution of bubble morphology when the surrounding aqueous medium is modified with varying concentrations of ethylene glycol.

**Comparison of the Two Systems.** As noted earlier, the wetting behavior of the silicon oil (PDMS) droplet system deviates significantly from that of the air bubble system. Although the air bubble contact angle increases monotonically with increasing EG concentration in the mixture, the silicon oil droplet exhibits a distinct response. At low EG concentrations, the silicon oil droplet maintains a constant contact angle. However, at a critical EG concentration, a wetting transition occurs, followed by a further increase in the contact angle with increasing EG concentration. This difference is surprising because the interfacial tensions remained constant or changed similarly with increasing EG concentration in both systems. Specifically, the air/HTS and PDMS/HTS interfacial tensions, which represent the interaction between the droplets/bubbles and the substrate, remained constant because EG molecules are not supposed to diffuse from the mixture and adsorb at these interfaces. Additionally, the air/mixture and PDMS/mixture interfacial tensions, which reflect the interaction between the droplets/bubbles and surrounding mixture, decreased similarly with increasing EG concentration (Figure 1). Based solely on these observations, one might reasonably anticipate a comparable change in shape (angular reconfiguration) for both the air bubbles and silicon oil droplets.

However, this simplistic view neglects the crucial role of the
contact angle for the pure water environment in surface force
balance. As discussed in the Section Theoretical Background,
the contribution of air/mixture or PDMS/mixture interfacial
tension variations to the imbalanced force used to predict the
evolution of the contact angle depends on the cosine of the
equilibrium contact angle (θ<sub>e</sub><sup>0</sup>) in the virgin state (i.e., pure
water). Thus, the smaller this angle, the greater is the value of
the interfacial component projected in the plane through cos
θ<sub>e</sub><sup>0</sup>. Consequently, for a similar decrease in the bubble/mixture
or droplet/mixture interfacial tension (|dγ<sub>air/M</sub>| or |dγ<sub>PDMS/M</sub>|),
a higher contact angle (θ<sub>e</sub><sup>0</sup>) in the case of an air bubble
(resulting in a lower cosine value) leads to a smaller decrease
in the in-plane projected interfacial tension compared to a
system with a lower contact angle. This situation is
schematically depicted in Figure 7. As can be seen in the

<figure><img src="image_14.png" /><figcaption>**Figure 7.** Schemes illustrating the force imbalance at the triple line: (a) air bubble and (b) silicon oil droplet. In this figure, the angle inside the air bubble corresponds to 180°—CA by convention. The variations in the interfacial tensions, $|d\gamma_{HTS/M}|$, $|d\gamma_{air/M}|$ and $|d\gamma_{drop/M}|$ are identical in these figures. The in-plane projected interfacial tension between the air bubble and mixture, $|d\gamma_{air/M}| \cos \theta_e^0$, and between the drop and mixture, $|d\gamma_{air/M}| \cos \theta_e^0$, are sketched at the bottom of each figure for comparison. Their difference in length illustrates the influence of the magnitude of the contact angle in the virgin state (i.e., pure water).</figcaption></figure>

figure through the length of the arrows representing the
interfacial tensions, the influence of the contact angle ($\theta_e^0$) for
pure water is observable in the resulting tension after being
projected in-plane. In the case of the air bubble, a higher
contact angle ($\theta_e^0$) translates to a shorter projected interfacial
tension (Figure 7a) compared with a lower contact angle in the
case of the silicon oil droplet (Figure 7b), $|d\gamma_{air/M}| \cos \theta_e^0 < |d\gamma_{PDMS/M}| \cos \theta_e^0$. This results in a stronger imbalanced force,
which leads to higher droplet retraction.

This larger imbalance might reduce the influence of the
potential difference between the equilibrium contact angle, on
which our theoretical predictions are based, and the static
contact angle observed experimentally. It is possible that when
the imbalance becomes sufficiently pronounced, the impact of
this discrepancy diminishes, making the predicted evolution of
the system appear closer to reality. In contrast, for the oil
droplet, the variations in the interfacial forces, $|d\gamma_{HTS/M}|$ and $|d\gamma_{PDMS/M}| \cos \theta_e^0$, are different yet very close. This subtle
difference could highlight the discrepancy between the static
and equilibrium contact angles, potentially explaining why a
strict application of the theoretical model does not fully
account for the stable contact angles observed experimentally.

■ CONCLUSION

This study investigated the impact of ethylene glycol (EG) on the wetting behavior of air bubbles and silicon oil (PDMS) droplets immersed in EG-water (EG-W) mixtures. Our results revealed distinct wetting behaviors between air bubbles and silicon oil droplets exposed to EG-W mixtures. While air bubbles exhibit a monotonic increase in the contact angle with increasing EG concentration, silicon oil droplets demonstrate a more complex response, maintaining a constant shape at lower EG concentrations before shrinking at higher concentrations.

This difference in behavior underscores the role of the
contact angle in the virgin state (i.e., pure water) to predict the

evolution of the morphology of the droplet or bubble after
increasing the EG concentration in the surrounding environ-
ment, through its influence on the in-plane projected interfacial
tension in the interfacial force balance. However, we caution
that the static contact angle observed experimentally may differ
from the equilibrium contact angle described by the Young
equation. Although the use of HTS self-assembled monolayers
on smooth silicon substrates aimed to minimize this
discrepancy, the interplay between the static and equilibrium
contact angles may still influence the observed behavior,
particularly for the silicon oil droplets. In the case of air
bubbles, the larger imbalance in interfacial forces reduces the
potential impact of this discrepancy, allowing the model to
more closely predict the observed evolution of the system. In
contrast, for the silicon oil droplets, the smaller imbalance in
interfacial forces might emphasize the limitations of the
theoretical model in fully explaining the experimental
observations.

Our findings offer insights into contact angle-dependent
processes, such as droplet or bubble behavior in confining
fluids, with potential applications in interfacial tension
manipulation. The sensitivity of bubble shape to EG
concentration suggests opportunities for controlling bubble
behavior in applications such as bubble removal or spreading.
Conversely, the relative stability of the silicon oil droplet
contact angle in EG-W mixtures for low EG concentrations,
highlights its potential for precise manipulation in cosolvent
environments, such as microfluidics.

■ ASSOCIATED CONTENT

● Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.langmuir.4c03817.

Complete data set (XLSX)

■ AUTHOR INFORMATION

Corresponding Author

Laurent Vonna – Institut de Science des Matériaux de Mulhouse (IS2M) CNRS, Université de Haute-Alsace, Université de Strasbourg, 68057 Mulhouse, France;

orcid.org/0000-0003-1764-1691;

Email: laurent.vonna@uha.fr

Authors

Philippe Kunemann – Institut de Science des Matériaux de Mulhouse (IS2M) CNRS, Université de Haute-Alsace, Université de Strasbourg, 68057 Mulhouse, France

Mohammed Djamel Bendimerad – Institut de Science des Matériaux de Mulhouse (IS2M) CNRS, Université de Haute-Alsace, Université de Strasbourg, 68057 Mulhouse, France

Thibault Ngadande Nerum – Institut de Science des Matériaux de Mulhouse (IS2M) CNRS, Université de Haute-Alsace, Université de Strasbourg, 68057 Mulhouse, France

Olga Voytko – Institut de Science des Matériaux de Mulhouse (IS2M) CNRS, Université de Haute-Alsace, Université de Strasbourg, 68057 Mulhouse, France

Manon Guyot – Institut de Science des Matériaux de Mulhouse (IS2M) CNRS, Université de Haute-Alsace, Université de Strasbourg, 68057 Mulhouse, France

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.langmuir.4c03817

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work is dedicated to the memory of Hamidou HAIDARA, whose pioneering efforts initiated this research. This work of the Interdisciplinary Institute HiFunMat, as part of the ITI 2021–2028 program of the University of Strasbourg, CNRS, and Inserm, was supported by IdEx Unistra (ANR-10-IDEX-0002) and SFRI (STRAT'US project, ANR-20-SFRI-0012) under the framework of the French Investments for the Future Program. T.N.N., O.V., M.G. are students of the National College of Chemical Engineering of Mulhouse, France (ENSCMu), who strongly contributed to the experimental work.

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