Thermocapillary Actuation of Liquids Using Patterned Microheater Arrays
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Thermocapillary Actuation of Liquids Using Patterned Microheater Arrays Joseph P. Valentino, Anton A. Darhuber, Sandra M. Troian, and Sigurd Wagner Departments of Electrical and Chemical Engineering, Princeton University Princeton, NJ 08544, U.S.A. ABSTRACT We demonstrate a microfluidic actuation technique capable of directing nanoliter liquid samples on the surface of a glass substrate through the use of both electronically addressable heater arrays and chemical patterning. Pathways for liquid movement are delineated by the arrangement of microheaters, which also provide the thermocapillary actuating force. The drops are confined to these pathways by a selectively deposited fluorinated monolayer, which defines the channel edges. Operating voltages in the range of 2-3 V is used to move, split, and trap liquids. This fluid transportation technique enables direct access to liquid samples for handling and diagnostic purposes and offers a low power alternative to existing microfluidic systems. INTRODUCTION Recent progress in the development of microfluidic analysis systems has the potential to revolutionize the fields of chemical, biological, and material science. By reducing sample size, advantages such as improved response times, reduced cost per analysis, and increased experimental throughput are made possible. In these devices, various fluid actuation techniques have been implemented for liquid flow in closed channels [1,2,3] and on open surfaces [4,5,6]. We present a microfluidic actuation technique that utilizes programmable surface temperature distributions in combination with a patterned non-wetting monolayer to control the direction and flow rate of nanoliter volumes of liquids on a free surface [7]. This system takes advantage of the increased role of surface tension in fluids with large surface area to volume ratios, by allowing one to induce flow via the manipulation of liquid-gas surface energy. Benefits of this fluidic actuation method include low voltage operation and the ability to handle polar and nonpolar liquids. EXPERIMENTS Theory A liquid film which is heated locally at some position x reduces the surface tension, γ (x), at that point and gives rise to a gradient in surface tension across the liquid. A thermocapillary shear stress is induced which pulls the liquid away from the heated region, τ = dγ/dx = (dγ/dT) (dT/dx)
(1)
where τ is the shear stress, and T is temperature [8,9,10]. For a thin flat liquid film, the flow speed is given by v (x) = h (x,t) τ / 2µ (x)
(2)
Downloaded from https://www.cambridge.org/core. University of Texas Libraries, on 09 Jan 2020 at 06:45:09, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-773-N10.3
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where h (x,t) is the film thickness, µ(x) is the local viscosity, and t is time. Sample Fabrication A cross-section through one micro-heater of the completed device is shown in Fig. 1(b). The heaters are fabricated over a 6.25 cm2 area of 0.7 mm thick glass slides. Organic co
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