4. Snake-like motion on surface with scales of varying lateral orientation
5. Inchworm-like motion on a surface with many scales
6. Multiple pulses in a soft gel moving on a highly frictional substrate
Video showing movements of water drops resulting from the condensation of steam on a gradient surface, the backside of which is cooled to ~ 14 deg C. This radial gradient of 1 cm diameter was prepared on silicon surface by diffusion controlled silanization. There are strong indications (Daniel, S., Sircar, S., Gleim, J., Chaudhury, M. K., Langmuir, 20, 4085-4092, 2004) that these fast movements result from the biasing of the randomly coalescing drops on the surface by the gradient of the surface energy. Note that the drops slow down as soon as the steam is turned off. This kind of passive removal of water drops from a surface could enhance its heat transfer efficiency when used in micro-heat exchangers and heat pipes.
Video showing movements of water drops resulting from the condensation of steam on a vertical surface that has a periodic gradient of surface energy. Here, the condensing drops move sidewise by surface energy gradient, collect in the hydrophilic channels and ultimately drain downward by gravity. This kind of combined action of surface energy gradient and gravity can be used to enhance the efficiency of heat exchangers having large surface area.
A water drop is confined between a gradient (lower) and a hydrophobic (upper) surface. The drop at first does not move due to hysteresis. However, as the upper substrate is vibrated vertically (here it is 1 Hz), the drop moves towards the region of higher energy. When the drop is squeezed, its contact angle tends to be larger than the advancing angles at both the left (i.e. towards high surface energy) and right (i.e. toward the lower surface energy) edges. However as the advancing contact angle at the left edge is smaller than the right edge, the former experiences a greater uncompensated force and advances toward higher wettability, whereas the right edge remains pinned. During the reverse stroke, the right edge experiences a greater uncompensated force; therefore, it retracts whereas the left edge remains pinned. The successive pinning and depinning of contact lines cause a net
of the drop toward the more wettable portion of the gradient.
simple fluidic device that operates on the principle of rectified motion of
confined drops being vibrated vertically. The lateral channel has a surface energy
gradient from west to east. The gradients of the other channels are from south
to north meeting the first one. When shape fluctuation is initiated, drop in
the first channel moves from west to east and coalesces with another drop
moving northward. The coalesced drop then moves farther east and coalesces with
another drop moving northward. This sequence is repeated.
Two water drops are placed on a hydrophobic silicon wafer that is vibrating laterally with a sawtooth wave of frequency of 70 Hz. Each drop experiences an asymmetric inertial force and moves on the surface. The large and the small drops move in opposite directions due to a difference in phase. They eventually collide and coalesce; the resultant larger drop continues to move to the right.
Demonstration of migration and mixing of liquid drops on a prototype fluidic device aided by lateral vibration having a sawtooth waveform. Three water drops were brought together from different areas on a PDMS coated glass slide, which has been embossed to confine the drops within predefined channels. The drops mix and cycle through a serpentine channel toward a detection area. If each drop contained a reaction component, for example, a candidate drug, a collection of target cells, and nutrients, these could be brought together, mixed and given sufficient time to react before arriving at the detection zone. A serpentine channel allows a certain residence time for the reaction to complete, and the drop could span several temperature zones to simulate various reaction conditions.
Created on Thu Aug 14 11:45:06 EDT 2003 on Lehigh University's Web Server.
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Last Modified: Feb 28 2007.