Microfluidic Actuation by Thermocapillary Forces: Fundamentals, Devices and Sensing Arrays

Lyman Handy Colloquium SeriesPresents Sandra Troian Professor of Applied Physics, Aeronautics and Mechanical Engineering. Caltech, Pasadena CA 91125. Abstract:
Liquid elements with dimensions in the micron to nanometer range manifest exceedingly large surface to volume ratios and are therefore highly susceptible to flow induced by surface stresses. This feature has been used to direct the motion of small, free surface liquid structures for micro-, bio- and optofluidic applications. Both normal and tangential stresses can be used to steer, mix, meter or shape liquid structures on demand. When such structures exhibit an effective zero Reynolds number and small aspect ratio, then inertial forces and phase lag are negligible and the liquid responds instantaneously to boundary stresses. Any time dependence of the flow is then strictly due to actuation of the bounding surfaces. These limits constitute the so-called slender gap approximation used here to investigate thermocapillary actuation of liquid elements with the potential for direct-write of 3D nanostructures. This possibility arises from analysis of several experiments conducted during the past decade in which molten nanoscale polymer films subject to an ultra large transverse gradient undergo spontaneous formation of nanopillar arrays. The formation of these self-assembling protrusions has been attributed to a Casimir-like radiation pressure caused by interfacial reflections of acoustic phonons. We demonstrate instead that thermocapillary stresses play a crucial if not dominant role in this formation process. Simulations of the governing interface equation, used to specify the pillar spacing and time-dependent height, are used to explore construction of nanoscale components for optical and photonic applications.

Location: Olin Hall of Engineering (OHE) - 122

Audiences: Everyone Is Invited

Contact: Petra Pearce Sapir