When 3:30 PM - 5:00 PM Nov 14, 2014
Where 1670 Beyster Building
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Superhydrophobic Surfaces: From Droplet Motion to Drag Reduction

Jonathan P. Rothstein
University of Massachusetts Amherst, Department Mechanical and Industrial Engineering

Superhydrophobic surfaces are hydrophobic surfaces with micron or nanoscale roughness.  These surfaces have a number of unique properties that we will exploit to control the motion of drops and the flow of fluid.  In the first part of the talk, we will discuss the use of superhydrophobic surfaces a platform for open microfluidics.  We will demonstrate that superhydrophobic surfaces are an excellent platform for droplet positioning, moving droplets, droplet coalescence and mixing.  Our superhydrophobic surfaces are created by imparting random roughness to Teflon through sanding with different grit sandpapers. With this technique, it is possible to create surfaces with similar advancing contact angles near 150°, but with a contact angle hysteresis that can vary smoothly from 3° to 50°.  We will focus on a number of experimental observations pertaining to drop dynamics along a surface with uniform hysteresis, drop motion along surfaces with transition zones from one hysteresis to another, and the collision of droplets on surfaces of uniform hysteresis. In the second part of the talk, we will show through a series of experiments and direct numerical simulations that superhydrophobic surfaces can produce significant drag reduction for both laminar and turbulent flows of water through channels and past cylinders using superhydrophobic surfaces.  These surfaces will be fabricated in a number of different ways including precisely patterning PDMS with well-defined micron-sized patterns of ridges or posts.  The key to drag reduction is that the surface roughness supports a shear-free air-water interface.  We will show that by precisely engineering these surfaces, drag reductions up to 75% and slip lengths up to 150μm can be obtained.  Drag reduction is shown to increases with post/ridge spacing, the fraction of air-water interface, and, in turbulent flows, with the Reynolds number.  Finally, if there is time we will demonstrate how the presence of slip can alter the shedding dynamics from bluff bodies.

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