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.
Highlights
As the application areas of microfluidics expand from the traditional mechanical engineering domain to more complex chemical and biological systems, various flow patterns and phenomena that could be favorably used at the reduced scale have been actively studied
Thermocapillarity as the effect of temperature-induced surface tension gradient on flow physics of the liquid, may lead to different forms of instabilities depending on the relative direction between temperature gradient to the liquid interface
If temperature gradient is perpendicular to the surface of the liquid, the thermocapillary instability is of the form of Marangoni convections and can be categorized into either linear or nonlinear steady basic states
Summary
As the application areas of microfluidics expand from the traditional mechanical engineering domain to more complex chemical and biological systems, various flow patterns and phenomena that could be favorably used at the reduced scale have been actively studied. The capillary action based on the change of surface or interfacial tension of liquid in contact with solid could be manipulated with electric field, magnetic field, thermal gradient as well as chemical concentration gradient. Various terms such as electrocapillary, electrotaxis, magnetocapillary, magnetotaxis, thermocapillary, thermotaxis, chemocapillary and chemotaxis have been used to describe these observations and experiments. Variousmethods computational methods have been flow patterns inside bubbles, droplets and liquid films subject to heat transfer. Thermocapillary-based used to capture flow patterns inside bubbles, droplets and liquid films subject to heat transfer.
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