John Christy

Experimental investigation of self-induced thermocapillary convection for an evaporating meniscus in capillary tubes using micro-particle image velocimetry

The present paper reports an experimental investigation of the self-induced liquid convection for an evaporating meniscus in small capillary tubes. The strong evaporative cooling at the triple contact line leads to a variation in temperature along the liquid–vapor interface, which generates a gradient of surface tension that in turn drives the observed convection. Ethanol and methanol in three tube sizes (600, 900, and 1630 μm) were investigated in this study. The flow pattern in the liquid phase has been characterized using a micro–particle image velocimetry (PIV) technique with a vector spatial resolution of 640 nm. Thermocapillary Marangoni convection is observed in horizontal diametrical sections of the horizontally oriented capillary tubes as two contrarotating vortices of similar strength, whereas in vertical diametrical sections a single clockwise vortex is mostly present. This distortion of the flow pattern could be attributed to gravity. The distortion and loss of symmetry in the vertical section...

Experimental and numerical investigation of the evaporation into air of a drop on a heated surface

Understanding the wetting and evaporation behaviour of volatile droplets on heated surfaces is very important for many industrial applications. In this paper the behaviour of a sessile drop evaporating on a heated surface is investigated both experimentally and numerically. Results are reported for the evaporation of water drops on two different substrates at various temperatures. A numerical model, based on a finite element method, has been developed to describe the hydrodynamics inside the evaporating drop and the effect of the humidity on the evaporation process, assuming the droplet to be a spherical cap. The energy and Navier–Stokes equations are solved within the droplet and the vapour concentration is computed using the diffusion equation. The drop volume and flow and temperature fields within the drop are obtained and the evolution of the volume in time is compared with the experimental results.

Prospects of confined flow boiling in thermal management of microsystems

This paper presents a review of prospects of confined flow boiling in future thermal management of microsystems such as microelectronics, optoelectronics, and microreactors. With the trend towards miniaturisation, heat removal has become the major bottleneck in microsystem development. In view of this we briefly discuss available cooling strategies, then assess studies of confined flow boiling and potential applications in heat dissipation from microsystems. Challenging issues related to implementation of the technique are addressed. It is concluded that confined flow boiling will be an attractive option for microsystem cooling. However, primary targets such as high critical heat flux and a stable, predictable nucleate boiling regime need to be achieved before this technique can be used in practice.

A Study of the Velocity Field during Evaporation of Sessile Water and Water/Ethanol Drops

Many studies have investigated evaporation of sessile drops in an attempt to understand the effect of wetting on the evaporation process. Recently interest has also increased in the deposition of particles from such drops, with evaporative mass flux being deemed to be responsible for ring-like deposits, and counteraction of the mass flux by Marangoni convection explaining more uniform deposition patterns. Understanding of such deposition processes is important in biological applications, such as the Litos test-system endorsed by the Russian Ministry of Health for diagnosis of urolithiasis and the evaporation of colloidal drops for depositing and organizing proteins and DNA. In most cases where deposition from evaporating drops has been studied, velocity information is inferred from the final deposition pattern or from mathematical modeling based on simplified models of the physics of the evaporation process. In this study we have directly measured the flow velocities in the base of sessile drops, using micro particle image velocimetry, viewing the drop from below, through the cover slide. For water drops, a radial pattern of flow was observed with a maximum velocity close to but not at the pinned outer edge. For ‘azeotropic’ ethanol/water mixtures, the velocity field is more chaotic to begin with, passing through a phase involving three or four recirculation cells and finally having the same radial pattern as for water drops.

Investigation of Flow Distribution in Microchannels Heat Sinks

The thermal efficiency of microchannel-based heat sinks relies on uniform fluid flow distribution between channels. Maldistribution, whether caused by poor manifold design or blockage of individual microchannels, can lead to hotspots and consequent thermal damage. This work considers design of manifolds for even flow distribution and the effect of channel blockage on the flow. An approximate model was used to evaluate the effect of manifold geometry on the flow distribution, and the results were compared with computational fluid dynamics (CFD) simulation. Various parameters, which influence the flow distribution such as the shape of distributing and collecting manifolds and position of inlet and outlet holes, have been studied for different inlet flow rates. The effect of channel blockage on flow distribution and pressure drop has been investigated. It was found that good agreement between results of the approximate model and results of CFD simulations are shown only for low Reynolds numbers. Results obtained by approximate model and CFD simulations were used to assist design of manifolds for uniform flow distribution between microchannels.

Effect of micropillar spacing and temperature of substrate on contact angle dynamics

ABSTRACT Contact angle dynamics of droplets deposited on a structured surface were studied in this work and the effects of substrate microstructure and temperature were investigated. Microstructures consisting of uniformly-sized, cubic micropillars with varying pillar spacings were constructed by microfabrication. Droplets (of the order of tens of microlitres in volume) were deposited on these surfaces and dynamic contact angles were observed using various techniques. Advancing and receding contact angles were measured using tilting of the surfaces or by injection and aspiration of fluid from a horizontal droplet by syringe. Droplets on these surfaces appeared to be mainly in the Wenzel state. Contact angle hysteresis was obtained as a function of pillar spacing or, equivalently, surface roughness. Depinning force was deduced and a linear dependence on maximal three phase contact line was found. The techniques of tilting the surface on which the droplet was deposited and uniformly increasing and reducing the volume of the droplet via the syringe both gave the same contact angle hysteresis for a given micropillar spacing. The effect of temperature was then assessed using a heated tilting plate. Contact angle hysteresis was found to increase with temperature. Further work to elucidate mechanisms governing this dependence will be undertaken.