A study of mixing in thermocapillary flows on micropatterned surfaces

Abstract

The recent introduction of actuation mechanisms for microfluidic transport based on free surface flows raises a number of interesting questions involving efficient mixing configurations, especially in systems with small aspect ratios. This work investigates the characteristics of convective and diffusive mixing in continuous-mode streaming of thermocapillary microflows on chemically micropatterned surfaces. Mixing times and mixing lengths relevant to chemical microreactors or gas sensors are investigated for various geometries and parameter ranges. Scaling arguments and full numerical solutions are presented to extract optimal operating conditions. Confocal fluorescence microscopy measurements of the interfacial diffusive broadening in adjacent flowing streams confirm numerical predictions. Three important mixing regimes, based on analogues of purely diffusive dynamics, Rhines-Young shear-augmented diffusion and Taylor-Aris dispersion are identified and investigated for use in free surface flows with large surface-to-volume ratios.

Marangoni convection in droplets on superhydrophobic surfaces

“…Finally, it is notable that, in addition to heat transfer applications discussed herein, the Marangoni convection discussed in this work may be exploited to enhance micromixing in fluid droplets (Darhuber et al 2004) and possibly as original microbiological assays (Chang & Velev 2006).…”

Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation

“…Some of the more popular techniques for propelling and manipulating liquid structures in open microfluidic devices include electrowetting, 1-6 dielectrophoresis 7 and thermocapillary actuation. [8][9][10][11] Extending the capabilities of these devices to include, for example, automated control over liquid positioning, size and composition, however, will require the integration of miniaturized sensors with fluidic chips.…”

“…This method of actuation has successfully been used to mobilize, split, coalesce, trap and even mix liquid samples. [8][9][10][11] The actual trajectories of the liquid streams or droplets is further refined by chemical treatment of the glass substrate to prevent undesirable lateral or streamwise spreading. This combination of thermofluidic actuation and surface chemical patterning allows numerous programmable fluidic functions to be carried out precisely and efficiently.…”

Principles of Microfluidic Actuation by Modulation of Surface Stresses

“…Addressable heating arrays can also be used to apply combinations of streamwise and transverse thermal gradients for efficient mixing in microscale geometries. Darhuber et al (2004) also examined convective mass transfer in thermocapillary flows on patterned substrates. Three important mixing regimes, based on analogues of purely diffusive dynamics, Rhines-Young shear-augmented diffusion, and Taylor-Aris dispersion, were investigated for possible use in microscale devices.…”