Session L34: Micro/Nano scale Flows: Interfaces I

Liquid-liquid phase separation within a co-axial flow system

In this work, we study liquid-liquid phase separation between two partially miscible streams, inside a co-axial glass capillary microfluidic device. A jet of poly(ethylene glycol) diacrylate (PEGDa) and glycerol is flowed in a continuous phase of a highly concentrated glycerol solution. As the two-streams flow downstream along the length of the channel, structures near the fluid-fluid interface evolve due to liquid-liquid phase separation, triggered by transport of water from the inner stream towards the outer stream. By tuning both flowrate and the initial composition of the system, we quantify the size, wavelength, and orientation of the structures that evolve as a function of position downstream along the length of the channel. Furthermore, via photopolymerization of the PEGDa rich jets at different positions along the length of the channel, we generate microfibers with different textures. Understanding the dynamic process of liquid-liquid phase separation within controlled flows may have important implications for design of materials with unique functionalities.   

Modeling hydrodynamic slip via Taylor dispersion

Hydrodynamic slip is a ubiquitous phenomenon in systems with fluid-solid interfaces, and it is non-negligible when the slip length is not small compared to other length-scales of the problem. In this work, we present a model for hydrodynamic slip as a function of effective diffusivity that leverages the phenomenon of Taylor dispersion. We validate this model against extensive molecular dynamics (MD) simulations of Lennard-Jones chains of varying lengths. We close by highlighting the key advantages of this framework for slip, and we describe a quasi-predictive method for estimating slip at moderate-to-high fluid shear rates.   

Plastron restoration by gas injection through super-hydrophobic surface created on porous material

In this work, we experimentally studied the plastron restoration by gas injection through super-hydrophobic surface created on a porous material . The SHS was created by sprayed coating a commercial super-hydrophobic coating on a porous steel plate. We found that the surface area being restored with a plastron increased with increasing gas injection pressure and gas injection duration. Using high-speed imaging, we found that the plastron restoration process involved bubble formation, merging and detachment. A layer of gas was left on the surface after bubble detachment. The size of the detached bubble increased with time due to bubble merging, and became stable when there was no more bubble merging. Increasing gas injection pressure led to higher gas flow rates, larger detached bubble sizes and faster plastron restorations. A plastron restoration within 0.3 s was achieved at the highest pressure, faster than the in-situ gas generation methods. Furthermore, we found that the gas flow rate through the underwater SHS can be described by a modified Darcy’s law. Our results highlighted the potential of using porous material and gas injection to restore the plastron and made possible the implementation of SHS in high-Reynolds number turbulent flows.   

Spontaneous emulsification of microfluidic droplets

We experimentally study the dynamics of spontaneous emulsification using microfluidic droplets made of a mixture of oil and solvent in a hydrophilic continuous phase. The spontaneous nucleation and subsequent coalescence of miniature drops within microfluidic droplets is investigated at different time-scales to better characterize capillary and mass transfer phenomena. We examine, in particular, the evolution of turbidity as a function of flow rates and fluid properties during the formation of microfluidic droplets in coaxial microdevices. We delineate the cross-over between diffusive and non-diffusive flow regimes and analyze the evolution of periodic microfluidic flow patterns based on solvent concentration. This work shows the possibility to manipulate the microflow behavior of ternary mixtures at ultralow interfacial tension.   

Extracting oil from an oil and water mixture by using their different wetting properties via the Acoustowetting phenomenon

In our experiments, we place drops of nano-emulsions that are stabilized by surfactants––SDS or Tween 20––atop a solid substrate; thin, micron-thick, oil films appear within minutes atop the sessile drops. Otherwise, the emulsions remain stable for over a year in their container. The large surface area of a small––10 μl––drop containing a kinetically stable oil in water nano-emulsion therein catalyzes the formation of a thermodynamically favorable low energy oil film at the drop boundary and especially at its free surface far before the emulsion destabilizes in the bulk of the mixture. We extract the micron-thick oil film appearing at the emulsion drops’ boundaries using a MHz-frequency Rayleigh type surface acoustic wave (SAW) in the solid substrate. This is the Acoustowetting phenomenon: Lower energy liquids––Oil––are attracted by the SAW to dynamically wet the substrate, leaving the emulsion drop behind, while higher energy liquids––Water––remain still.

 

Moreover, a balance between SAW induced acoustic radiation pressure and emulsion pressure within the drop pushes the emulsion away from the source of the SAW, leading to a non-intuitive extraction of oil from the drop surface normal to the SAW path. During oil extraction, the initially white-shaded emulsion drops become increasingly transparent owing to the reduction in oil content therein down to approximately a third of their initial 10-50% concentration.   

Forced Phase Separation in a Closed Cell

Phase separation of oil-water mixtures is of interest as water recovery becomes more pertinent. It is therefore of interest to find ways to accelerate this process to make applications such as graywater recycling more efficient. Preliminary work carried out by the group of Manor at Technion suggests that a MHz Rayleigh surface acoustic wave (SAW) propagating through the underlying substrate has the ability to accelerate spinodal decomposition in the mixture sitting atop. Experiments are performed using a lithium-niobate substrate and a drop of silicon oil and water mixture. In this study, we present a simplified model derived from first principles that describes a binary mixture undergoing spinodal decomposition due to a substrate vibration in a closed cell. Enforcing conservation of mass for the mixture and requiring the model to be consistent with the second law of thermodynamics provides the governing equations of the system. The model is then closed with an appropriate free energy functional, which we do in accordance with the Cahn-Hilliard theory. Numerical results carried out in 3D are presented with parameter values estimated from experimental observations.  Simulations show that phase separation is accelerated by such a force in agreement with experimental results.   

Thin Films Under Action of Surface Acoustic Waves: Experiments, Modeling, and Simulations

This talk focuses on modeling and computations of experiments involving silicon oil films spreading under the action of surface acoustic waves. Specifically, we focus on the films driven by MHz-frequency Rayleigh surface acoustic waves (SAWs) and their association with Eckart streaming: a mass transport mechanism induced by sound or ultrasound wave attenuation within the liquid film. While three different acoustic mechanisms can cause streaming (Stokes drift, acoustic radiation pressure, and Eckart streaming), our particular focus is on Eckart streaming, which becomes increasingly significant for thicker films. Within long-wave approximation, we arrive at a variant of the thin film equation with an additional SAW term used to describe film dynamics. Finally, We discuss the comparison of experimental and computational findings.   

Instability and rupture of sheared viscous liquid nanofilms

Nanoscale liquid films resting on solid surfaces often occur in nature and in industrial processes, and can rupture due to intermolecular forces. We numerically study the evolution and rupture of viscous nanometric films, incorporating the effects of surface tension, van der waals forces, thermal fluctuations and viscous shear. We show that thermal fluctuations create perturbations that can trigger film rupture, but they do not significantly affect the growth rate of the perturbations. The film rupture time can be predicted from a linear stability analysis of the governing thin film equation, by considering the most unstable wavelength and the thermal roughness. Furthermore, applying a sufficiently large unidirectional shear can stabilise large perturbations in a two-dimensional film, but does not inhibit rupture in three-dimensional films, as perturbations are not suppressed in the direction perpendicular to the applied shear. However, if the direction of shear varies in time, the growth of large perturbations is prevented in all directions, and rupture can hence be impeded.   

Salt-mediated non-monotonic transport of surfactant-laden oil droplets in confined spaces

We report our observation of a non-monotonic motion of oil droplets covered with various surfactants upon exposure to a single, unidirectional ionic solute gradient. Under the influence of high-salinity water, the oil droplets surrounded by anionic surfactants (sodium dodecyl sulfate, SDS) aggregate and either stay motionless or rapidly migrate against the expected direction of the well-studied diffusiophoresis. Meanwhile, the droplets covered by non-ionic surfactants (Triton X-100) remain emulsified and immobile. Noticeably, the droplets covered in a mixture of these two surfactants exhibit a strong diffusiophoresis migration. In many cases, the droplets that are far from the high salinity source start to swim randomly after a few minutes. These observations suggest the existence of competition during the droplet locomotion under confinement, which is between diffusiophoresis and, in this case, Marangoni propulsion triggered by the salted-out SDS. We further experimentally confirm this investigation by characterizing the dynamic interfacial tension of oil droplets in aqueous salt solutions. Our study offers a potential strategy for enhanced oil recovery in high salinity conditions.   

Laminar Drag Reduction in Microchannels with Quasi-liquid Slippery Polymer Brush Surfaces

In laminar flows in microchannel, drag reduction is capable of reducing the required pumping power in microchannel, enhancing microfluidic cooling, augmenting electrokinetic energy harvesting, and enabling better flow control in chemical and biological devices. One important passive drag reduction method is to modify the wetting property of the microchannel surfaces, such as textured superhydrophobic and slippery liquid-filled surfaces. However, there are two significant challenges associated with textured slippery surfaces: 1) the surface coverage of fluid lubricant layer is often limited, restricting overall drag reduction; and 2) fluid lubricant suffers from poor durability. In recent studies, a novel type of non-textured slippery quasi-liquid polymer brush surfaces have been developed. This novel non-textured liquid-like slippery surface represents a promising candidate for drag reduction. In this work, we fabricated microchannels with slippery  polymer brush grafted surfaces and microchannels made by silicon surfaces. To investigate the drag reduction induced by the slippery polymer brush surfaces, we measured the pressure drops along the microchannels under various flow rates and Reynolds numbers. The drag reduction coefficient and corresponding fluid slip length are calculated. The drag reduction coefficient between 10% to 15% and fluid slip length in micron scale are achieved.

    

Influence of Surface Roughness on Lubricated Contact of Soft Surfaces

The Stribeck curve, which measures the coefficient of friction against the dimensionless sliding speed, is notably different for patterned surfaces compared with smooth surfaces (Peng et.al, 2021, Nat. Mater.). The elastohydrodynamic flow behind this observation is not entirely understood. Here, we introduce a model for the lubricated contact of soft surfaces that are locally patterned but globally cylindrical. We solve the resulting system of equations numerically for various geometrical parameters that characterize the surface patterns. Additionally, we examine how the properties of the fluid layer and the soft solid affect the Stribeck curve. At low sliding speeds, where the fluid film is thin, the lubricated flow treats each asperity as a distinct degenerate contact. Conversely, at high sliding speeds, where the fluid film is thicker than the typical depth of the asperity, the entire patterned surface can be approximated as a smooth parabola. Using the method of multiple scales and perturbation analysis, we obtain an analytical solution for the coefficient of friction at high sliding speeds and demonstrate its consistency with our numerical results. This study offers a quantitative understanding of friction in the contact of soft wet objects and lays the groundwork for incorporating the friction coefficient into haptic signals in robotics and haptic engineering.   

The effect of miscibility on the nanoscale dynamics of wetting

Wetting and dewetting dynamics span multiple scales, with nanoscale viscous bending of interfaces and liquid-solid friction impacting rheology up to the macroscale. Our study examines how miscibility influences friction at the three-phase contact line (contact line friction). Large-scale molecular dynamics simulations are employed to explore wetting regimes and molecular interfacial effects, which would be extremely challenging to inspect experimentally. We focus on silica-like surfaces to capture hydrophilic interactions with polar liquids like water. The first simulation set we present examines the spreading dynamics of aqueous glycerol droplets. Glycerol concentration is varied to mimic viscosity adjustment in experiments. Existing theoretical and semi-empirical correlations fail to predict the observed scaling between viscosity and contact line friction. Our results are explained by extending Molecular Kinetic Theory to liquid mixtures and considering interfacial effects such as adsorption and depletion. In the second set, confined water-butanol meniscus are examined. Partial miscibility leads to a low-tension diffuse interface between the two liquid phases. We aim to quantify the impact of mixing at the interface on solid friction and compare molecular simulations with Cahn-Hilliard Navier-Stokes equations.