Thermocapillarity in Microfluidics—A Review

Abstract

This paper reviews the past and recent studies on thermocapillarity in relation to microfluidics. The role of thermocapillarity as the change of surface tension due to temperature gradient in developing Marangoni flow in liquid films and conclusively bubble and drop actuation is discussed. The thermocapillary-driven mass transfer (the so-called Benard-Marangoni effect) can be observed in liquid films, reservoirs, bubbles and droplets that are subject to the temperature gradient. Since the contribution of a surface tension-driven flow becomes more prominent when the scale becomes smaller as compared to a pressure-driven flow, microfluidic applications based on thermocapillary effect are gaining attentions recently. The effect of thermocapillarity on the flow pattern inside liquid films is the initial focus of this review. Analysis of the relation between evaporation and thermocapillary instability approves the effect of Marangoni flow on flow field inside the drop and its evaporation rate. The effect of thermocapillary on producing Marangoni flow inside drops and liquid films, leads to actuation of drops and bubbles due to the drag at the interface, mass conservation, and also gravity and buoyancy in vertical motion. This motion can happen inside microchannels with a closed multiphase medium, on the solid substrate as in solid/liquid interaction, or on top of a carrier liquid film in open microfluidic systems. Various thermocapillary-based microfluidic devices have been proposed and developed for different purposes such as actuation, sensing, trapping, sorting, mixing, chemical reaction, and biological assays throughout the years. A list of the thermocapillary based microfluidic devices along with their characteristics, configurations, limitations, and improvements are presented in this review.

Effect of surface-active contaminants on radial thermocapillary flows

“…Current scientific challenges involving thermocapillary flows are focused on microscopic scales [6]. Indeed, the prevalence of interfacial phenomena as the size decreases allows for a fine control of free surface flows.…”

Effect of temperature gradient on the cross-stream migration of a surfactant-laden droplet in Poiseuille flow

“…Studies on the droplet motion in the presence of external effects such as electric (Ahn et al 2006;Link et al 2006;Bandopadhyay et al 2016;Mandal et al 2016), magnetic (Seemann et al 2012), temperature (Karbalaei et al 2016) and acoustic fields (Seemann et al 2012) are gaining much importance nowadays due to the ease with which these fields can be applied in respective applications. The presence of these fields induces an imbalance in stresses at the droplet interface and modifies the net force acting on the droplet which in turn alters the droplet velocity and associated flow field.…”

Lehmann rotation of cholesteric droplets driven by Marangoni convection

“…It is well known that the Marangoni effect can cause the thermomigration of the droplets providing that the latter are free to migrate (for a review, see for instance Ref. 32 ). This is the case if the temperature gradient is applied parallel to the glass plates.…”