Peichun Amy Tsai

Evaporation-triggered wetting transition for water droplets upon hydrophobic microstructures.

When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and suggest the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.

Control of slippage with tunable bubble mattresses

Tailoring the hydrodynamic boundary condition is essential for both applied and fundamental aspects of drag reduction. Hydrodynamic friction on superhydrophobic substrates providing gas–liquid interfaces can potentially be optimized by controlling the interface geometry. Therefore, establishing stable and optimal interfaces is crucial but rather challenging. Here we present unique superhydrophobic microfluidic devices that allow the presence of stable and controllable microbubbles at the boundary of microchannels. We experimentally and numerically examine the effect of microbubble geometry on the slippage at high resolution. The effective slip length is obtained for a wide range of protrusion angles, θ, of the microbubbles into the flow, using a microparticle image velocimetry technique. Our numerical results reveal a maximum effective slip length, corresponding to a 23% drag reduction at an optimal θ ≈ 10°. In agreement with the simulation results, our measurements correspond to up to 21% drag reduction when θ is in the range of −2° to 12°. The experimental and numerical results reveal a decrease in slip length with increasing protrusion angles when θ ≳ 10°. Such microfluidic devices with tunable slippage are essential for the amplified interfacial transport of fluids and particles.

Microscopic structure influencing macroscopic splash at high Weber number

The dynamics of water drop impact at high impinging velocity onto superhydrophobic substrates is experimentally investigated. The solid substrate—comprised of regular and hydrophobic micropillars—is transparent, thereby facilitating close-up, top-or-bottom-view, high-speed imaging. With a sufficient impact velocity, instead of a completely-bouncing ‘‘Fakir’’ droplet, wetting splashing can occur, with an entrapped air bubble at the centre surrounded by a wetted area as well as an emission of satellite droplets during the advancing phase of spreading lamella. A large portion of the lamella travels upon air and subsequently recoils due to surface tension, forming a partial rebound on the central wet spot. We present and discuss quantitative results of the entrapped air bubble, the central wetted area, and the maximal spreading lamella as the impact velocity is increased. We further vary the lattice periodicity of the micro-patterns and find its profound influence on the macroscopic flow. More specifically, directional splashing can emerge, emitting secondary droplets in certain directions which are associated with the lattice. Directional splashing can be suppressed to a gentle spreading by decreasing the periodicity of the lattice and, furthermore, can be tuned to a completely-wetting splashing in the diagonal directions of the lattice by a larger periodicity, offering opportunities to control the wetting process. Finally, the elimination of directional splashing by reducing air pressure suggests that the underlying air is squeezed outwards by the falling droplet upon the solid boundary whereby the air flow is affected, leading to different splashing behavior.

Dynamics of microvortices induced by ion concentration polarization

We investigate the coupled dynamics of the local hydrodynamics and global electric response of an electrodialysis system, which consists of an electrolyte solution adjacent to a charge selective membrane under electric forcing. Under a dc electric current, counterions transport through the charged membrane while the passage of co-ions is restricted, thereby developing ion concentration polarization (ICP) or gradients. At sufficiently large currents, simultaneous measurements of voltage drop and flow field reveal several distinct dynamic regimes. Initially, the electrodialysis system displays a steady Ohmic voltage difference (ΔV_{ohm}), followed by a constant voltage jump (ΔV_{c}). Immediately after this voltage increase, microvortices set in and grow both in size and speed with time. After this growth, the resultant voltage levels off around a fixed value. The average vortex size and speed stabilize as well, while the individual vortices become unsteady and dynamic. These quantitative results reveal that microvortices set in with an excess voltage drop (above ΔV_{ohm}+ΔV_{c}) and sustain an approximately constant electrical conductivity, destroying the initial ICP with significantly low viscous dissipation.

Formation and prevention of fractures in sol–gel-derived thin films

Sol-gel-derived thin films play an important role as the functional coatings for various applications that require crack-free films to fully function. However, the fast drying process of a standard sol-gel coating often induces mechanical stresses, which may fracture the thin films. An experimental study on the crack formation in sol-gel-derived silica and organosilica ultrathin (submicron) films is presented. The relationships among the crack density, inter-crack spacing, and film thickness were investigated by combining direct micrograph analysis with spectroscopic ellipsometry. It is found that silica thin films are more prone to fracturing than organosilica films and have a critical film thickness of 300 nm, above which the film fractures. In contrast, the organosilica films can be formed without cracks in the experimentally explored regime of film thickness up to at least 1250 nm. These results confirm that ultrathin organosilica coatings are a robust silica substitute for a wide range of applications.

Momentum and mass transport over a bubble mattress: the influence of interface geometry

In this study we investigate numerically how a partially slippery bubble mattress can be employed to enhance momentum and mass transport. The bubble mattress, which is a superhydrophobic surface, consists of an array of slippery gas bubbles with no-slip walls in between. We consider a pressure-driven laminar flow over the bubble mattress, with a solute being supplied from the gas bubbles. The results show that, because of effective wall slip, solute transport can be enhanced significantly with respect to a fully saturated no-slip wall. The enhancement depends on the interface geometry of the bubble mattress, i.e. on the bubble size, protrusion angle, and surface porosity. In addition, we demonstrate that the mass transfer enhancement disappears below a critical bubble size. The effective slip vanishes for very small bubbles, wherefore interfacial transport becomes diffusion dominated. For large bubbles, solute transport near the interface is convection enhanced.

Altering Emulsion Stability with Heterogeneous Surface Wettability

Emulsions–liquid droplets dispersed in another immiscible liquid–are widely used in a broad spectrum of applications, including food, personal care, agrochemical, and pharmaceutical products. Emulsions are also commonly present in natural crude oil, hampering the production and quality of petroleum fuels. The stability of emulsions plays a crucial role in their applications, but controlling the stability without external driving forces has been proven to be difficult. Here we show how heterogeneous surface wettability can alter the stability and dynamics of oil-in-water emulsions, generated by a co-flow microfluidic device. We designed a useful methodology that can modify a micro-capillary of desired heterogeneous wettability (e.g., alternating hydrophilic and hydrophobic regions) without changing the hydraulic diameter. We subsequently investigated the effects of flow rates and heterogeneous wettability on the emulsion morphology and motion. The experimental data revealed a universal critical timescale of advective emulsions, above which the microfluidic emulsions remain stable and intact, whereas below they become adhesive or inverse. A simple theoretical model based on a force balance can be used to explain this critical transition of emulsion dynamics, depending on the droplet size and the Capillary number–the ratio of viscous to surface effects. These results give insight into how to control the stability and dynamics of emulsions in microfluidics with flow velocity and different wettability.

Spatial Site-Patterning of Wettability in a Microcapillary Tube

Substrate functionalization is of great importance in successfully manipulating flows and liquid interfaces in microdevices. Herein, we propose an alternative approach for spatial patterning of wettability in a microcapillary tube. The method combines a photolithography process with self-assembled monolayer formation. The modified microcapillaries show very sharp boundaries between the alternating hydrophilic/hydrophobic segments with an achieved smallest domain dimension down to 60 μm inside a 580 μm inner diameter capillary. Our two-step method allows us to pattern multiple types of functional groups in an enclosed channel. Such structures are promising regarding the manipulation of segmented flows inside capillaries.

Observation and experimental investigation of confinement effects on ion transport and electrokinetic flows at the microscale

Electrokinetic effects adjacent to charge-selective interfaces (CSI) have been experimentally investigated in microfluidic platforms in order to gain understanding on underlying phenomena of ion transport at elevated applied voltages. We experimentally investigate the influence of geometry and multiple array densities of the CSI on concentration and flow profiles in a microfluidic set-up using nanochannels as the CSI. Particle tracking obtained under chronoamperometric measurements show the development of vortices in the microchannel adjacent to the nanochannels. We found that the direction of the electric field and the potential drop inside the microchannel has a large influence on the ion transport through the interface, for example by inducing immediate wall electroosmotic flow. In microfluidic devices, the electric field may not be directed normal to the interface, which can result in an inefficient use of the CSI. Multiple vortices are observed adjacent to the CSI, growing in size and velocity as a function of time and dependent on their location in the microfluidic device. Local velocities inside the vortices are measured to be more than 1.5 mm/s. Vortex speed, as well as flow speed in the channel, are dependent on the geometry of the CSI and the distance from the electrode.

Effect of temperature on foamability using a thermoresponsive polymer

We experimentally examine foam dynamics and foamability using a thermoresponsive polymer, Poly(n-isopropylacrylamide) or PNIPAM. Under a constant gas injection, our results show that temperature significantly affects the foam height produced. A stable foam was observed at room temperature while unstable at temperature above the lower critical solution temperature. Analysis of the foam structure reveals an important increase in the coalescence rate at high temperature, thereby suggesting a change of the air-water interface properties with the temperature. The findings demonstrate the controlling ability of the PNIPAM polymer to produce thermoresponsive foams, offering beneficial applications for foaming processes and mineral flotation.We experimentally examine foam dynamics and foamability using a thermoresponsive polymer, Poly(n-isopropylacrylamide) or PNIPAM. Under a constant gas injection, our results show that temperature significantly affects the foam height produced. A stable foam was observed at room temperature while unstable at temperature above the lower critical solution temperature. Analysis of the foam structure reveals an important increase in the coalescence rate at high temperature, thereby suggesting a change of the air-water interface properties with the temperature. The findings demonstrate the controlling ability of the PNIPAM polymer to produce thermoresponsive foams, offering beneficial applications for foaming processes and mineral flotation.