title: Thermocapillary flow in glass tubes coated with photoresponsive layers.
abstract: Thermocapillary flow has proven to be a good alternative to induce and control the motion of drops and bubbles in microchannels. Temperature gradients are usually established by implanting metallic heaters adjacent to the channel or by including a layer of photosensitive material capable of absorbing radiative energy. In this work we show that single drops can be pumped through capillaries coated with a photoresponsive composite (PDMS + carbon nanopowder) and irradiated with a light source via an optical fiber. Maximum droplet speeds achieved with this approach were found to be ~300 μm/s, and maximum displacements, around 120% of the droplet length. The heat generation capacity of the coatings was proven having either a complete coating over the capillary surface or a periodic array of pearls of the photoresponsive material along the capillary produced by the so-called Rayleigh-Plateau instability. The effect of the photoresponsive layer thickness and contact angle hysteresis of the solid-liquid interface were found to be important parameters in the photoinduced thermocapillary effect. Furthermore, a linear relationship between the optical intensity I(o) and droplet velocity v was found for a wide range of the former, allowing us to analyze the results and estimate response times for heat transfer using heat conduction theory.
authors.affiliation: Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México , Apdo. Postal 70-360, México D.F. 04510, México.
authors.forename: J Rodrigo
authors.initials: JR
authors.lastname: Vélez-Cordero
authors.forename: A M
authors.initials: AM
authors.lastname: Velázquez-Benítez
authors.forename: J
authors.initials: J
authors.lastname: Hernández-Cordero