Session A29: Microscale Flows: Emulsions & Microscale Flows: Interfaces and Wetting

Predicting Uniform Droplet Formation in Microchannels Using Time-Domain Analysis

The merge of two droplets at a T-junction in a microfluidic device causes small fluctuations in the flow which leads to a cascading effect that induces perturbations and missed mergers. We study the effect of flow fluctuations and inlet geometry on microfluidic drop mergers by utilizing numerical simulations and in-depth time domain analysis of two-phase fluid flow velocities. Furthermore, we demonstrate the successful use of time-domain signal decomposition to predict events downstream by monitoring the signal changes at a single position over time. The variational mode decomposition method utilized is specifically suited for non-linear and non-stationary signals as opposed to the traditional methods for signal decomposition such as Fourier transform and wavelet transform. This method allows us to decompose the signal into components that can be used to eliminate noise of certain frequencies in real-time and predict dynamics in the device. We propose metrics to predict the formation of monodisperse or satellite droplets by analyzing the time-domain signal within the inlet and prior to the droplet pinch-off.   

Tilting prevents the collapse of flexible sheets during imbibition

We study the imbibition of a wetting liquid between flexible sheets that are fixed in both ends. The liquid flow contracts sheets that are soft and forms a narrow neck that slows down the filling of the channel. Below a threshold stiffness, the sheets collapse and imbibition stops. Using the finite difference method, we solve the lubrication equation coupled with slender body deformation, and find that the meniscus motion has two regimes. Before reaching halfway, it advances faster than in traditional imbibition in stiff sheets. After, it slows down as the sheets contract and increase the hydraulic resistance of the channel. We propose a formula for the filling time in the asymptotic limit prior to collapse. In microfluidics, collapsing of flexible channels is a concern encountered in several applications. We show that tilting the initially parallel sheets cures this problem, and fills the channel in a short duration.   

Effect of wettability on crude oil recovery using microfluidics

Surface wettability plays an essential role in drop impact dynamics, microfluidic emulsion dynamics, slip flow for drag reduction, fluid-fluid displacement in various technological applications. Targeting enhanced oil recovery applications, we experimentally investigate the effect of porous networks' wettability on the invasion morphology and sweep efficiency of viscous oil displaced by different salinity floods using microfluidics. Two different microfluidic models are employed, namely a uniform network of homogeneous pore arrangement and a rock network mimicking a sandstone structure.  Systematical experiments (of the same flood-pairs) are carried out in both hydrophilic and hydrophobic chips for comparison. The results show that regardless of flood type or wettability, oil recovery is lowered by approximately 20 – 40% in the rock-like porous media compared to the uniform one.  We also find that the hydrophilic microfluidic rock has a remarkable increase in oil recovery by ≈ 30%, compared to the hydrophobic case. In addition, we observe a more pronounced lateral growth of displacing pattern of the aqueous flood when the surface is hydrophilic. Finally, by incorporating the contact angle into analysis, we quantitatively explain the increasing factor in both recovery rate and finger width for the hydrophilic vs. hydrophobic rock-liked porous networks.   

Biphasic co-flow through a sudden expansion or contraction of a Hele-Shaw channel

In the present talk we present the study of biphasic co-flow through a Hele-Shaw cell with an abrupt change, either expansion or contraction, in channel width. Assuming gentle variation of the flow in the streamwise direction (valid at small capillary number), we derive the 3^rd nonlinear ordinary differential equation (ODE) governing the shape of the interface separating the two fluids. The interfacial profiles obtained by solving this nonlinear ODE are further used as the initial guess for the interface shape in the rigorous numerical solution of the two-dimensional free-boundary problem by the finite element method. The theoretical results are then compared with the experimental findings showing a good agreement. The amplitude of the capillary ridge emerging upstream from a sudden expansion (for moderate expansion ratios up to 1:3) and leading to narrowing of the thread of the disperse phase is significant, however not large enough to trigger its instability and breakup.   

Diffusion-driven directional wetting transitions

We report on directional wetting transitions in gas-entrapping microtextured surfaces (GEMS) submerged in water. GEMS comprise SiO₂/Si surfaces adorned with arrays of microscale doubly reentrant cavities (DRCs) that have been demonstrated to entrap air underwater regardless of their chemical make-up. Here, we investigated time-dependent wetting transitions of water into DRCs as a function of hydrostatic pressure and compared them with simple cylindrical cavities. Experiments revealed that the cavities in the center sustained air-filled Cassie-states for significantly longer times than the boundary cavities, even though they had identical dimensions, surface chemistry, and liquid entry pressure. In fact, cavity filling always advanced boundary inwards, and the lifetimes of inner side cavities could be tuned to vary dramatically based on cavity size, pitch, surface density, and water column height. We will explain these findings based on our analytical calculations and computational simulations. Insights from this work will aid in creating coating-free surfaces capable of retaining air underwater over longer durations.   

Molecular desorption by a moving contact line

The surface properties at the molecular scale influence the contact angle of a macroscopic sessile drops [1, 2]. Therefore, the contact line is very sensitive to these properties which are at the origin of the hysteresis. We address the fundamental questionof a possible action of the contact line on the surface, at molecular scale by desorbing molecule from the solid surface.

To answer this question, we use atomic force microscopy (AFM) as used before with good results [3]. Thanks to this tools, nanoneedles (with a radius between 60 and 500 nm) are repeatedly dipped in droplets of 6 liquids and the capillary force which appears with the formation of a meniscus is measured.

As the contact line sweeps the nanoneedle surface, the capillary force increases by yielding to a more "hydrophilic" surface. We interpret this effect by the desorption atmospheric contaminants on the solid surface and show that the capillary force at the contact line is at the origin of the mecanism. The dynamic does not change with the liquid or the solid surface but the magnitude is very sensitive with the surface tension of the liquid. Our results open new outlooks on the comprehension of molecular effects at the contact line [4].   

Deformation dynamics of a thin liquid film driven by dielectrophoretic forces

The controlled deformation of thin liquid films can be leveraged toward the creation of adaptive optical elements. We here present a combined theoretical and an experimental study of the dynamics of a thin liquid film subjected to a dielectric force distribution created by surface electrodes.  We model the spatial electric field produced by a pair of parallel electrodes and calculate the Maxwell stresses on the interface. These stresses serve as boundary conditions for the fluids' momentum equations, resulting in an evolution equation which we approximate under the longwave approximation. For validation of our theoretical framework, we developed a holography-based experimental setup that enables high frame-rate measurements of microscale deformations. We actuate the liquid using different patterns of microfabricated electrodes at the surface of the fluidic chamber and different time-dependent actuation functions and observe its temporal response.   

Phase separation of a Ionic Liquid mixture by heating locally in a microfluidic system

In recents works, we have shown that heating locally (50 -100 µm) in a microfluidic system involves two mechanisms: i) locally the PDMS is deformed and leads to a thickness gradient which generates a local capillary pressure; ii) the temperature profile generated by a local resistance leads to surface tension gradients and thus to stresses along an interface between two immiscible phases, generating flows close to this interface.

We have shown that local heating in a microfluidic cavity can generate the phase separation of an ionic liquid of the LCST (low critical solution temperature) type into two separate phases: one rich in water, the other rich in ionic liquid. This system allows to consider an alternative to the recycling of these solutions which have many applications, e.g. in green chemistry. More precisely, we study the phase separation along the phase diagram of the solution composition. Three regimes are observed depending on the mass fraction of ionic liquid. These regimes display complex 3D flows combining the different idendified mechanisms that are rationalized. Interestingly, a thermocapillary-instability appear at intermediate ionic liquid mass fraction.   

Convective mitigation of dendrite growth

The growth of metallic dendrites during the electrodeposition and solidification of metal films is a challenging scientific and industrial problem. To date, studies show conflicting evidence as to whether the introduction of flow into such systems prevents dendritic growth. We isolate the contribution of flow to dendritic growth by considering a far-from-equilibrium limit of both electrodeposition and solidification processes. This limit does not support any other stabilizing mechanisms, such as surface tension and the Gibbs-Thomson effect, making it a worst-case scenario. We give a physical model and employ the perspective of kinetic stability to show that ion convection is unequivocally preferable for the mitigation of dendrites compared to the transport of ions solely by diffusion. Convective flow disperses ion concentration gradients near dendrites, thereby inhibiting the auto-catalytic action diffusion has on dendrite growth. The mitigation effect is more effective at higher surface roughness, or equivalently, at more densely packed dendritic hotspots. We conclude by providing a design strategy for convective deposition systems.