How to Make Water Run Uphill
HARVARD UNIV CAMBRIDGE MA DEPT OF CHEMISTRY
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A surface having a spacial gradient in its surface free energy was capable of causing drops of water placed on it to move uphill. This motion was the result of an imbalance in the forces due to surface tension acting on the liquid-solid contact line on the two opposite sides uphill or downhill of the drop. The required gradient in surface free energy was generated on the surface of a polished silicon wafer by exposing it to the diffusing front of a vapor of decyltrichlorosilane. The resulting surface displayed a gradient of hydrophobicity with the contact angle of water changing from 97 deg. to 25 deg. over a distance of 1 centimeter. When the wafer was tilted from the horizontal plane by 15 deg., with the hydrophobic and lower than the hydrophilic, and a drop of water 1 to 2 microliters was placed at the hydrophobic end, the drop moved toward the hydrophilic end with an average velocity of about 1 to 2 mmsec. In order for the drop to move, the hysteresis in contact angle on the surface had to be low or 10 deg.
- Physical Chemistry
- Fluid Mechanics