(283c) Simultaneous Flow and Drying in Open Microchannels

Microchannels have applications in microfluidic devices, micromolding, and flexible electronic devices. Liquid flow in open microchannels is controlled by capillarity, which draws the liquid down the channel, and viscous forces, which slow the flow. When a solid coating is desired on the interior of the channel, as in the case of flexible electronic devices, liquid inks that dry and solidify as they flow down the channels are used. In this work, we investigate the balance between flow and drying during capillary-driven flow in open microchannelsâ??a topic that has received little attention in the literature to-date. Using high speed visualization, we study the influence of drying on the evolution of capillary flow dynamics over time. Specifically, we investigate aqueous polyvinyl alcohol (PVA) solutions imbibed into photolithographically created open microchannels with widths of 10 â?? 200 µm and depths of 50 µm. As an evaporating liquid travels down a microchannel, we show that the increase in viscosity (specifically the local increase at the contact line) reduces the total travel distance of the liquid and also has a unique influence on the flow dynamics. Specifically, strong departures from the generally accepted Lucas-Washburn theory of capillary flow are revealed. The highly dynamic flow field in the vicinity of the contact line during drying is also investigated using micro-particle image velocimetry. Pinning induced by the high-viscosity contact line is shown to disrupt the well-characterized fountain flow exhibited by liquids in microchannels. Comparisons are made to PVA solutions flowing in a high humidity environment, where evaporation is severely suppressed. This work has important applications in all processes involving the spreading of volatile solutions or suspensions, especially printing processes requiring the patterning of inks in microchannels or other confined volumes.