| dc.description.abstract | Thermocapillary phenomenon is caused by the temperature difference inside the liquid, which makes the surface tension gradient, and then generates traction in a specific direction. In previous researches, process and operation of driving droplet with thermocapillary method is simple and cost lower, but insufficient driving force in short distance causes droplet migrate slowly. Therefore, improves droplet migration speed by changing surface properties or combining other driving methods.
In the design, based on surface physical modification can improve the droplet migration, using dry-etching to fabricate columnar microstructure arrays with different pitches on the silicon wafer. Establish a side wall around the microstructure area confining the droplet and make it appear in a regular capsule shape, be expected to reduce the effects of changes in the shape of the droplet in migration. In the operation, utilizing prewetting the structural gaps by liquid film to avoid volume loss during droplet migration on gaps.
After the temperature in hot and cold zones of test piece is stable, it is found that a critical structural gap distance exists by comparing the migration of droplet on test pieces with different structure intervals. Decreasing the temperature gradient when the gap distance is smaller than the critical distance will increase the migration ability, and vice versa when it is larger than the critical distance. From the mechanical analysis of droplet, found that the droplet is mainly affected by negative viscous resistance, positive Marangoni driving force, and unstable internal capillary reflow. In addition, there is a force inside the structural gap which will transfer the fluid inside the gap from high temperature zone to low temperature zone, but the transmission capacity will be suppressed by droplet migration speed. As the structure gap distance is increased, both the shape of droplet and the internal temperature distribution will become more distorted, resulting in greater influence of calculation errors and unconsidered forces.
From the viewpoint of fixed structure spacing, the thermal capillary force has a positive correlation with the temperature gradient. And the continuous extension of the droplet span during the migration process has just little effect on the thermal capillary force. From the viewpoint of fixed temperature gradient, increasing the structure gap distance will reduce the droplet migration effect under low temperature gradient conditions, and increase the droplet migration effect under high temperature gradient conditions. When the gap distance above the critical value, the migration capacity if affected by the contact angle more than the droplet span.
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