Applications of superhydrophobic surfaces for drag reduction
Microfluidics and nanofluidics
Lab on chip systems
Electrokinetic transport and fluid dynamics
at electrochemical surfaces such as ion exchange membranes, electrodes or micro/nano-channel intersections.
In systems including aqueous salt solutions subject to electric fields, electrokinetic transport is often described by simplified ohmic models neglecting nonlinearities arising from the coupling of electric fields with electric double layers. However these systems can present complex behavior at various experimentally relevant conditions. For example, at sufficiently high electric forcing, electrically charged boundary layers become hydrodynamically unstable. This instability can trigger (chaotic) electrokinetic flows (as shown below). This naturally occurring and recently discovered secondary flow is one of the key mechanisms of transport beyond diffusion limit and play a crucial role in ion mixing.
Drag reduction at slippery curved bubble surfaces
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. We have presented 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 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.
Controllable microfluidic bubble mattress with integrated gas and liquid channels, allowing for tunable bubble interface geometry. From left to right: the first is an optical image of the microfluidic device with integrated gas (G) and liquid (L) channels, the second is a scanning electron microscopy image of a representative microfluidic device, showing two main microchannels for gas (Pg) and liquid (Qw) streams connected by gas-filled side channels, third and forth are bright-field microscopy images of bubbles protruding into the liquid microchannel.
Dynamics of gas absorption at slippery and curved gas bubble surfaces
Interfacial phenomena, which is a rapidly growing interdisciplinary field with an enormous variety of applications ranging from bio/chemical engineering separation processes to engineered materials and development of energy, food, and environmental technologies. With these motivations, I have been experimentally and numerically investigating the transport processes near (i) soft gas-liquid interfaces and (ii) charged ion selective interfaces.
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Comparison of instantaneous contour plots of dimensionless anion concentration field superimposed with flow lines, obtained from 2D direct numerical simulations. Here the applied potential is 3 DC volts. Red color shows high concentration (~1) and blue shows low concentration (~0).
The dynamics of interfacial mass transfer of dissolved oxygen are analyzed numerically considering (a) kinetic equilibrium conditions at bubble surfaces that is conventionally described by Henry's Law and (b) non-equilibrium conditions at bubble surfaces using Statistical Rate Theory (SRT). Fluorescent lifetime measurements performed using an oxygen sensitive fluorophore dissolved in water flowing past oxygen gas bubbles. Measurements show that kinetic equilibrium is not established for short contact times. Mass transfer into water flow past micro-bubbles can be well described by the simulations performed at non-equilibrium conditions for short exposure times (~180 microseconds), deviating from the commonly accepted Henry's Law.
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