Session H7: Microfluids: Interfaces and Wetting II

Flow through a thin film on non-flat substrates

The ability of liquids to form films on surfaces is essential for many technical applications such as the coating of surfaces or the liquid transport. While for many heterogeneous materials it is possible to introduce effective material properties from spatial averaging, here close to the percolation transition the size of a representative domain becomes comparable to the size of the whole system. In this work we investigate the morphological evolution and the transport properties of a thin wetting layer adhering to an irregular rough substrate. Static film profiles are obtained by numerically minimizing the interfacial energy including a generic short ranged interface potential to account for a precursor film and a finite apparent contact angle. Assuming that the flow does not alter the profile of the liquid meniscus, we employ the static film configurations resulting from the energy minimization to solve the linearized steady thin film equation and obtain the total volume flux. Our results show that the connectivity between liquid domains plays a key role in predicting the transport properties of the liquid interface.   

Altering the Flow of Gas through Modification of Surface Films

Normally the flow of gas in a channel is considered to be a function of the pressure difference between the ends of the channel and the geometry. For high Knudsen numbers, the flow also depends on the tangential momentum accommodation coefficients (TMAC). Here we consider methods of altering the TMAC, and thus the flow of gas at 1 atm through a narrow channel, by the use of surface films that alter the TMAC. Gas flow was determined by measuring the damping on a glass sphere as a function of separation from a flat plate. The solids were coated with octadecyltrichlorosilane (OTS), which undergoes a melting-like transition near room temperature. The measured damping passes through a maximum in the temperature range of 9 - 42 $^{\circ}$C and thus the TMAC also passes through a maximum. We attribute this maximum to competing effects due to the decrease in surface roughness and the decrease in stiffness as a function of temperature. Control of flow via alteration of a surface film should also be possible using other methods of altering the state of surface films.   

Mass Transfer of Gas on Slippery Superhydrophobic Surface

Superhydrophobic substrates containing gas bubbles are advantageous for generating hydrodynamic slippage. When a viscous liquid flowing upon, bubble surfaces provide shear-free gas-liquid interfaces thereby slippage. Besides, the absorption of gas into the liquid occurs at the bubble surfaces. We experimentally measure and numerically estimate the mass transfer of gas absorption at the stable gas/liquid interfaces for short contacting times. We study the net rate of gas absorption experimentally by in-situ measurements of dissolved oxygen concentration profiles in aqueous solutions flowing over oxygen bubbles using fluorescent lifetime imaging microscopy. We numerically analyze the dynamics of interfacial mass transfer of dissolved oxygen, by considering (i) kinetic equilibrium conditions at bubble surfaces that is conventionally described by Henry's Law and (ii) non-equilibrium conditions at bubble surfaces using Statistical Rate Theory (SRT). Our experimental results show that kinetic equilibrium is not established for short contact times. Mass transfer of gas into liquid flow past micro-bubbles can be well described by our simulations performed with the non-equilibrium theory for short exposure time ($\sim$ 180 $\mu$s) of liquid with a microbubble, deviating from the commonly   

Drag Reduction using Superhydrophobic Sanded Teflon

In this talk, we present a series of microfluidic experiments designed to investigate drag reduction using series of roughened Teflon surfaces. The Teflon surfaces where made superhydrophobic by imparting surface texture through sanding with sand papers with a range of grit sizes. Our previous work showed that there exists an optimal sand paper grit (240 grit) for eliminating contact angle hysteresis. We will show that a Teflon surface roughened with the same sand paper grit also maximizes the drag reduction and the slip length observed in laminar flows. Increasing or decreasing the grit size was found to reduce the drag reduction and slip length. A number of different sanding protocols were investigated including sanding preferentially in the flow direction, normal to the flow direction and with a randomized circular pattern. Of these three techniques, sanding in the flow direction was found to maximize the slip length.   

Lubricant-impregnated surfaces for drag reduction in viscous laminar flow

For the first time, we explore the potential of lubricant impregnated surfaces (LIS) in reducing drag. LIS, inspired by the surface of the Nepenthes pitcher plant, have been introduced as a novel way of functionalizing a surface. LIS are characterized by extremely low contact angle hysteresis and have been show to effectively repel various liquids including water, oils, ketchup and blood. Motivated by the slippery nature of such surfaces, we explore the potential of LIS to reduce drag in internal flows. We observe a reduction in drag for LIS surfaces in a viscous laminar drag flow and model the impact of relevant system parameters (lubricant viscosity, working fluid viscosity, solid fraction, depth of texture, etc.).   

Flow-driven failure of liquid-filled surfaces

A micro-patterned surface impregnated with liquid is subjected to laminar shear flow and the subsequent drainage of the liquid is measured. Such lubricant-infused rough surfaces offer a promising new approach to drag reduction by providing surface slip between the mobile lubricant within the roughness and the outer flow in a manner that is more stable and robust than traditional air-layer-based super-hydrophobic surfaces. We consider the effect of roughness geometry and lubricant properties on the drainage behavior of liquid-infused surfaces in order to understand the physical mechanisms of liquid drainage and the failure modes of such drag-reducing surfaces at the micro-scale. The analysis of micro-scale drainage is then used to develop design criteria for enhanced lubricant retention and drag reduction under a variety of shear flow conditions.   

Molecular dynamics simulations of disjoining pressure effects in ultra-thin water films on a metal surface

Disjoining pressure, the excess pressure in an ultra-thin liquid film as a result of van der Waals interactions, is important in lubrication, wetting, flow boiling, and thin film evaporation. The classic theory of disjoining pressure is developed for simple monoatomic liquids. However, real world applications often utilize water, a polar liquid, for which fundamental understanding of disjoining pressure is lacking. In the present study, molecular dynamics (MD) simulations are used to gain insights into the effect of disjoining pressure in a water thin film. Our MD models were firstly validated against Derjaguin's experiments on gold-gold interactions across a water film and then verified against disjoining pressure in an argon thin film using the Lennard-Jones potential. Next, a water thin film adsorbed on a gold surface was simulated to examine the change of vapor pressure with film thickness. The results agree well with the classic theory of disjoining pressure, which implies that the polar nature of water molecules does not play an important role. Finally, the effects of disjoining pressure on thin film evaporation in nanoporous membrane and on bubble nucleation are discussed.   

Computational modelling of microfluidic capillary breakup phenomena

Capillary breakup phenomena occur in microfluidic flows when liquid volumes divide. The fundamental process of breakup is a key factor in the functioning of a number of microfluidic devices such as 3D-Printers or Lab-on-Chip biomedical technologies. It is well known that the conventional model of breakup is singular as pinch-off is approached, but, despite this, theoretical predictions of the global flow on the millimetre-scale appear to agree well with experimental data, at least until the topological change. However, as one approaches smaller scales, where interfacial effects become more dominant, it is likely that such unphysical singularities will influence the global dynamics of the drop formation process. In this talk we develop a computational framework based on the finite element method capable of resolving diverse spatio-temporal scales for the axisymmetric breakup of a liquid jet, so that the pinch-off dynamics can be accurately captured. As well as the conventional model, we discuss the application of the interface formation model to this problem, which allows the pinch-off to be resolved singularity-free, and has already been shown to produce improved flow predictions for related ``singular'' capillary flows.   

Computational and experimental investigation of capillary self-focusing in a microfluidic system

We present a capillary focusing method for generating monodisperse submicrometric droplets. The emulsification technique relies on an abrupt change in the aspect ratio of a single shallow and wide microchannel that merges into a deep reservoir [Appl. Phys. Lett. 88:024106 (2006)]. We present a computational framework, supported by experimental observation, to address the capillary self-focusing, in which the interface between the two fluids takes the shape of a tongue narrowing in the flow direction just ahead of the holding reservoir. Our numerical approach is based on a volume-of-fluid method for computing the interface motion and for modeling the surface tension in a Hele-Shaw flow. We present and compare numerical and experimental results for the width of the tongue and predict and measure the transition between two different emusification mechanisms occuring in this geometry.