Experimental and analytical study of the effect of contact angle on liquid convective heat transfer in microchannels G. Rosengarten a, * , J. Cooper-White b , G. Metcalfe c a School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney 2052, Australia b Division of Chemical Engineering, University of Queensland QLD, 4072, Australia c CSIRO, Melbourne 3190, Australia Received 2 November 2005; received in revised form 23 February 2006 Available online 9 June 2006 Abstract In this paper we examine the effect of contact angle (or surface wettability) on the convective heat transfer coefficient in microchan- nels. Slip flow, where the fluid velocity at the wall is non-zero, is most likely to occur in microchannels due to its dependence on shear rate or wall shear stress. We show analytically that for a constant pressure drop, the presence of slip increases the Nusselt number. In a micro- channel heat exchanger we modified the surface wettability from a contact angle of 20°–120° using thin film coating technology. Appar- ent slip flow is implied from pressure and flow rate measurements with a departure from classical laminar friction coefficients above a critical shear rate of approximately 10,000 s 1 . The magnitude of this departure is dependant on the contact angle with higher contact angles surfaces exhibiting larger pressure drop decreases. Similarly, the non-dimensional heat flux is found to decrease relative to laminar non-slip theory, and this decrease is also a function of the contact angle. Depending on the contact angle and the wall shear rate, vari- ations in the heat transfer rate exceeding 10% can be expected. Thus the contact angle is an important consideration in the design of micro, and even more so, nano heat exchangers. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Microfluidics; Slip flow; Micro heat transfer; Heat exchanger; Contact angle; Wetting 1. Introduction Fluid flow in microchannels has received much research interest due to its seemingly endless applications [1,2]. Up to now three main application areas have driven the research: (1) two-phase flow for ink jet printing which was probably the first microfluidic application; (2) high surface area heat sinks for high-flux computer chip cooling using multiple microchannel arrays [3]; and (3) lab-on-a- chip or micro total analytical systems that increase the speed and sensitivity of chemical processes, and often involve heating and temperature control of reagents and products [4]. The second and third application areas both involve convective heat transfer from a solid to a fluid, which is a process not well understood in microfluidic channels due to the relative dominance of surface forces over body forces. This fact has driven the recent increase in interest in convective heat transfer in microchannels, where a reliable heat transfer rate prediction method is required for efficient device design. For macro-scale heat transfer design there are numerous well-established correlations for the Nusselt number (non- dimensional heat transfer coefficient) that depend on the flow conditions. Recently however, there has been consid- erable debate in the literature over the applicability of these macro-scale Nusselt number correlations to microfluidic flow [5–10]. This is due, primarily, to the large variation in experimental data, which we have plotted in Fig. 1. 0017-9310/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2006.02.057 * Corresponding author. Tel.: +61 2 9385 4112; fax: +61 2 9663 1222. E-mail address: g.rosengarten@unsw.edu.au (G. Rosengarten). www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 49 (2006) 4161–4170