INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 14 (2004) 1185–1189 PII: S0960-1317(04)75808-8 W-shaped meniscus from thin polymer films in microchannels Kahp Y Suh 1 , Seungwoo Chu 2 and Hong H Lee 2 1 School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea 2 School of Chemical Engineering, Seoul National University, Seoul 151-742, Korea E-mail: sky4u@snu.ac.kr Received 9 February 2004, in final form 20 April 2004 Published 17 June 2004 Online at stacks.iop.org/JMM/14/1185 doi:10.1088/0960-1317/14/8/010 Abstract Transient meniscus formation in microchannels reveals that a ‘W’-shaped meniscus forms at early times for relatively large channel size and lower annealing temperature. This phenomenon is opposed to the uniformly curved meniscus, which has been believed to be dominant for transients. This formation is attributed to the competition between the Laplace pressure and viscous resistance. Conditions are given under which the W-shaped meniscus forms, which is shown to represent the experimental data satisfactorily. During transients, the contact angle is found to be maintained at its equilibrium value. 1. Introduction Meniscus formation is a classical phenomenon that is well described by Young’s law [1, 2]. When a liquid wets a wall, the Laplace pressure is generated due to the curved interface, which leads to the capillary rise of the liquid. Typically, the curvature-induced Laplace pressure is simply counterbalanced by the gravitational force. While capillarity has been studied extensively [3–6], capillarity in microchannels has not received much attention in spite of its importance in microsystems such as microanalysis systems, which involve fluid in microchannels, and micro sensors and devices where a fluid is used as a coolant. Much less well known is the transient behavior of meniscus formation in microchannels. The conventional capillary system involves a low- viscosity liquid such as water. The time it takes for such a liquid to form a fully developed meniscus is so short that it is almost next to impossible to observe a transient. When a polymer is used, on the other hand, a sufficient time is allowed for the observation due to its high viscosity or low mobility. The slow dynamics afforded by the use of a polymer was the reason for focusing our attention on the transient behavior of the meniscus formed from a polymer. To investigate the behavior, we used capillary force lithography (CFL) [7], which has recently been developed for patterning polymers on large areas. When an elastomeric mold such as polydimethylsiloxane (PDMS) is placed on the polymer film spin-coated onto a substrate and then heated above the glass transition temperature (T g ) of the polymer, the capillary force allows the polymer melt to fill up the void space between the mold and the polymer, thereby generating the negative replica when the mold is removed. In the experiment, the transients of the filling of the void channels in PDMS mold are investigated. 2. Experimental section We fabricated a PDMS (Sylgard 184, Dow Corning) mold that has void microchannels by casting PDMS against a master prepared by conventional photolithography. The master has protruding lines varying in width from 1 to 10 µm with an interval of 0.5 µm between the lines. Therefore, the PDMS mold has void channels with a width varying from 1 to 10 µm. Commercial polystyrene (PS) (M w = 1.4 × 10 5 , T g = 100 ◦ C) was used as purchased from Aldrich. Silicon wafer that was used as the substrate was cleaned by ultrasonic treatment in trichloroethylene and methanol for 5 min each and dried in nitrogen. Native oxide was not removed from the surface of the wafer and thus would exist on the surface. PS films were spin-coated onto the substrate under conditions (5 wt%, 3000 rpm for 35 s) that would lead to a thickness larger than 300 nm so as to exclude substrate effects. When the film thickness is less than 100 nm, intermolecular forces [8–10] 0960-1317/04/081185+05$30.00 © 2004 IOP Publishing Ltd Printed in the UK 1185