Session X18: Microscale and Nanoscale Flows: Interfaces, Wetting, Emulsions

Dynamics of a wet array of hinged plates

We present the nonlinear dynamics of side-by-side elastically hinged plates carrying drops in three configurations: trapped at the base, pinned at the tips, and sliding. We first examine the equilibria and motion of a single pair, and find that the collapsed state depends not only on the liquid or structural properties but also on the initial conditions. Then we consider an array of parallel plates at unstable equilibrium, where a disturbance amplifies while traveling at a constant speed and leaves behind a pattern of clusters. We derive expressions of the propagation speed and mean bundle size that agree with numerical results.   

Charge separation at contact lines slows down the breakup of liquid bridges on surfaces

Electrostatic charge separation at receding contact lines, called slide electrification, has been extensively studies in recent years.  The resulting electrostatic interactions fundamentally contribute to contact angle hysteresis and can significantly slow down the movement of sliding drops. Yet, the relevance of electrostatic effects in dynamic wetting beyond single drops has largely remained obscure. Here, we experimentally study the breakup of liquid bridges on non-conductive solid substrates and find a substantial influence of electrostatic charge separation. First, electrostatic interactions lead to the spontaneous random movement of satellite drops after the liquid bridge breakup. Second, electrostatic forces slow down the dynamics of the breakup process. We find that the influence of electrostatics increases with the liquid viscosity and that our experimental observations align with slide electrification theory. Our results highlight the wider importance of slide electrification in dewetting scenarios beyond drops, even when the liquid is connected to a large reservoir.   

Microfluidic multiphase flows of immiscible fluids with a miscible solvent in the continuous phase

We experimentally examine the behavior of ternary fluid systems in the presence of a miscible solvent in the continuous phase. Coaxial microdevices are employed to generate a range of microflow patterns made of water-in-oil and oil-in-water dispersions over wide variations of flow rates. In particular, we investigate the relationship between solvent concentration and segmented flows characteristics and reveal a range of intriguing interfacial behaviors at ultralow interfacial tension, including spontaneous emulsification, dynamic wetting transitions and phase inversion, as well as unstable jets. This work clarifies important aspects of hydrodynamic interactions between interfacial and diffusive phenomena at the small scale.   

Investigation on cluster formation and velocity distribution of evaporating molecules under visible light irradiation

Understanding and enhancing evaporation is important in developing many industrial applications, such as water desalination technologies and cooling devices. With the aim of precise estimation of evaporation and condensation mass flux at the liquid-vapor interface, many models have been proposed until now. However, most of them assume equilibrium flow at the liquid-vapor interface, although it is known that the evaporated molecules do not follow the Maxwell-Boltzmann distribution, which is the velocity distribution at equilibrium state. To solve this problem, we conducted a molecular beam experiment to obtain the accurate velocity distribution of the evaporated molecules using the time-of-flight method, and successfully confirmed the deviation from the Maxwell-Boltzmann distribution.

Additionally, evaporation under visible light irradiation has been attracting significant attention due to its exceptionally high evaporation rate. It has been hypothesized that in addition to conventional thermal evaporation, the energy of photons cleaves off water molecules in clusters. We aim to clarify the mechanism by detecting the molecular weight of emitted particles with a quadrupole mass spectrometer to examine whether the evaporated water molecules are forming a cluster or not.   

Effect of viscosity on liquid-liquid dispersions within milli-scale symmetric confined impinging jets

This work presents the formation of liquid-liquid dispersions in a symmetric confined impinging jets (CIJs) contactor, where two immiscible liquids enter from opposing directions into a mixing chamber. Both the inlet channels of the liquids and the outlet channels have dimensions in the micro or mm scale. The configuration leads to rapid mixing or emulsification of the liquids. CIJs find numerous applications such as extraction, rapid precipitation, and flash synthesis. They are favoured for the enhanced mass transfer and process intensification. However, the hydrodynamics at the impinging zone and the emulsification for symmetric CIJs are still not well explored.

The dispersion formation is studied both experimentally, with high-speed imaging, and numerically with computational fluid dynamics (CFD) simulations. Experimentally, the droplet size distribution was measured for different fluids. Water (viscosity of 1.38 mPa∙s) was used as the continuous phase, while kerosene (viscosity: 2.04 mPa∙s) and silicone oils (viscosities: 10 mPa∙s, 50 mPa∙s) were used as the dispersed phase. The results show that when kerosene is dispersed, the drop sizes follow a log-normal distributions and are significantly influenced by the energy dissipation rate in the impinging zone. With the more viscous liquids, the distributions shift to larger sizes, while bimodal distributions are observed. This is attributed to different Rayleigh-Plateau wavelengths and the contribution from satellite drops. Specific interfacial areas and the Sauter mean diameter are obtained at different operating conditions. Compared with asymmetric CIJs with only one outlet, it is found that the emulsions formed in symmetric CIJs have smaller droplet sizes and larger specific interfacial area at higher energy dissipation rates, suggesting that symmetric CIJs intensify the processes. The CFD simulations utilise a transition from Volume of Fluid to Discrete Phase Model to capture the mechanism of droplet formation from the initial continuous liquid films. The predicted drop sizes are found to agree well with the experimental measurements.   

High Fraction Nano-Emulsions: A Vapor Condensation Breakthrough

Nanoscale emulsions are crucial in the food and pharmaceutical industries, enhancing product stability and facilitating targeted drug delivery with improved efficacy[1]. Commercial emulsification relies heavily on the use of power-intensive homogenizers, which often produce sharp temperature changes that can degrade the quality of emulsions. The present work addresses these problems using a novel energy-efficient vapor condensation technique. Previous research using this technique has successfully generated a wide range of emulsions, including water-in-oil (W/O), oil-in-water (O/W), and Pickering emulsions [2-3]. However, these studies were limited to creating emulsions with a low dispersed phase fraction (<0.2), Φ_d = V_d/V_t, where V_d, V_t are the dispersed phase volume and total volume of the emulsion. Here, we have developed an intelligent system that offers an effective engineering solution to increase the Φ_d(> 0.8) significantly. Additionally, our innovative design allows for precise control over the growth of nucleated droplets, resulting in highly uniform, nanometer-scale emulsions. This advancement overcomes a significant limitation of the vapor condensation technique, paving the way for its commercialization and large-scale industrial applications   

Longevity of superhydrophobic surface in undersaturated liquid

We experimentally studied the longevity of superhydrophobic surface (SHS) when it exposed to undersaturated liquid. A wetting transition was induced due to the diffusion of gas from SHS to ambient liquid. SHSs with two types of texture geometries, holes and posts, fabricated on polydimethylsiloxane (PDMS) were studied. The status of gas on the SHS was observed by a non-intrusive optical technique based on light reflection. Due to the non-uniform diffusion rate across the surface, SHS longevity was determined based on the status of gas on the entire surface. We found that the SHS longevity (t_f) followed a scaling relation: t_f~(1−s)⁻², where s was the ratio of gas concentration in liquid to that in the plastron. This scaling relation implied that as the gas was dissolved into the liquid, the mass flux J reduced with time as: J~t^−0.5. Furthermore, we found that the diffusion length L_D reduced as the undersaturation level increased, following a scaling relation: L_D~(1−s)⁻¹. Last, we found that the SHS longevity in undersaturated liquid increased as increasing the texture depth and as increasing the thickness of the gas permeable material (here, the thickness of PDMS surface). Our results provide a better understanding of SHS longevity in undersaturated liquid.   

Monodispersed water-in-oil emulsion generation in electric field

Emulsification involves dispersing one liquid into another immiscible liquid, with water-in-oil emulsions being particularly valuable in applications like biological, pharmaceutical, cosmetics, and food industries. Large-scale high-shear mixers are commonly used for emulsification, but small-volume and more controllable microfluidic methods are preferred when uniform emulsions are needed. We present an electrical method to generate emulsions using simple parallel plates, a device similar to electromicrofluidic (EMF) platform that offers integrability and portability. By applying a moderate electric field, a water droplet in an oil environment will spread and begin to eject small droplets along its edge. By controlling the applied electric field, emulsions with uniform and adjustable volume can be produced. Key parameters like interfacial tension, frequency and amplitude of the applied electrical signal are characterized. The developed emulsification method offers a simpler way to generate water-in-oil emulsions and opens up new possibilities for industrial applications.   

Pore-scale effect on evaporation from a porous surface

Evaporation from porous surfaces often plays an essential role in nature and industrial applications such as comet sublimation, transpiration, micro pumps, and membrane-based cooling devices. Evaporation generates a layer with a highly nonequilibrium gas flow next to the liquid surface, and this layer is called the Knudsen layer. Understanding the microscopic transport phenomenon in the Knudsen layer is of great importance since it governs the evaporation mass flux. In this work, we analyzed evaporation flows from two-dimensional porous surfaces and clarified the pore-scale effect on the evaporation mass flux. To cover a wide range of pore scales (i.e., the pore spacing and diameter), we employed the low-variance deviational simulation Monte Carlo (LVDSMC) method, which can reduce the computational cost by typically two orders of magnitude for small Mach number conditions. The evaporation mass flux was evaluated for porous surfaces with the same pore opening fraction but different pore spacing. The simulation results showed that the evaporation mass flux increases as the pore spacing increases. To explain the pore scale dependence obtained from the numerical simulations, we examined the pressure distribution within the Knudsen layer. In addition, we constructed models for two limiting cases where the pore spacing and diameter are comparable or far larger than the mean free path of gas molecules.   

Influence of Surface-Active Agents on the Transport of Nanoparticles through Interfaces

Investigating the adsorption and desorption energy of nanoparticles (NPs) at oil-water interfaces, particularly in the presence of surfactants and/or NPs, is important for various applications.¹ Both surfactants and NPs have the potential to influence interfacial properties, so that careful consideration of the interplay between surfactants and NPs is essential when studying the free energy of the system for the desorption or adsorption process. Herein, we employed the Dissipative Particle Dynamics (DPD) method^2-4 to calculate the energy needed for single nanoparticle migration, encompassing both homogeneous and amphiphilic NPs (Janus NPs), across the oil-water interface. We considered scenarios involving NPs alone, surfactants alone, and their coexistence. Our results for NPs that traversed between the oil and water phases revealed that surfactants lowered the energy barrier for desorption. However, the formation of an NP layer at the interface reduced NP mobility and at high enough concetration at  the interface resulted in jammimg.  It was found that the creation of a bridge of water (or oil) as the NP moved out of the water phase (or the oil phase) led to deviations from theoretical predicitons of desorption. 

References

1.         S. Kaur and A. Roy, Environment, Development and Sustainability, 2021, 23, 9617

2.         T. X. Nguyen, et al., Polymers, 2022, 14, 543.

3.         T. X. Nguyen, et al. he Journal of Physical Chemistry B, 2022, 126, 6314

4.         T. X. Nguyen, et al., The Journal of Physical Chemistry B, 2024. 128(12), 3016

  

Enhancing Oil Recovery Using Carbonated Seawater-Based Waste Concrete Solution Flooding: A Microfluidic Approach

Chemical EOR is gaining more attention as the newly explored reservoirs have deep pore structures. This research introduces an environmentally benign chemical EOR technique using a novel flooding fluid composed of carbonated seawater and waste concrete. This post-carbon captured solution exhibits tunable properties like lower pH, salinity, and optimized concentrations of divalent ions such as calcium and magnesium. The sessile drop method measures the contact angles between the oil/flooding fluid and rock surface, which provides insights into the wettability characteristics essential for effective oil displacement. Subsequent experiments consist of polydimethylsiloxane (PDMS)-based microchannel with the modified surface mimicking carbonate and sandstorm reservoir surface to examine the flow behavior within pore throats and intersections. Lastly, a comparative analysis was conducted using a random pore network model to analyze the proposed solution's performance, unlike commonly used surfactant and polymer-based EOR fluids, such as sodium dodecyl sulfate and carboxymethyl cellulose. The results highlight the potential of this novel flooding fluid towards environmentally friendly EOR with storage of carbon.