Capillary Flow Dynamics in Composite Rectangular Microchannels with Rough Walls

In this article, we consider rectangular microchannels composed of glass and thin polymeric walls with different roughness in which opposed walls are of the same material but adjacent walls are not. We propose a model for fluid capillary transport into these rectangular microchannels when horizontally positioned and focus our research on how the microchannel aspect ratio modifies the motion during the initial viscous regimes. The model relies on an effective static contact angle and an effective friction coefficient that averages local magnitudes in the cross section. An extensive experimental investigation with different microchannels enabled us to obtain these coefficients for different aspect ratios. While for low aspect ratios, the effective contact angle presents the smallest values, the effective friction coefficient shows the larger ones. With rough surfaces, the spontaneous occurrence of pinning and depinning events associated with sharp wall defects notably reduces the effective static contact angle even when high aspect ratios are used. The obtained values of the effective friction coefficient show good agreement with previous literature investigations for rough and smooth lateral wall surfaces. Finally, we propose a non-dimensional time to establish when contact angle effects dominate the dynamics. We found that for the materials and fluid properties used in this work, these effects become negligible for times larger than t similar to 1 s.