Active control of flow over object surfaces achieved by means of mechanical and/or electrical methods has recently been studied. However, there has been no report on actively switching the surface drag of an object by chemical modification of the object's surface. Poly(N-isopropylacrylamide) (PNIPA) was grafted onto the surface of an aluminium (Al) substrate via (A) surface-initiated atom transfer radical polymerization and (B) radical polymerization with an azo-group surface initiator. The grafting density was 0.19 and 0.15 chains nm-2, respectively. The water contact angle of the PNIPA-grafted Al surface reversibly changed between 55° and 82° for (A) and between 42° and 65° for (B) at temperatures of 25 and 40 °C, which was ascribed to the temperature-responsive, hydrophilic-hydrophobic switching of the grafted PNIPA surface. The PNIPA grafting was applied on the surface of an ogive-shaped Al model. The normalized dropping speed of the model in water increased 1.1 times at 42 °C in comparison to that at 22 °C. Switching of the surface drag of PNIPA-grafted Al in water was demonstrated on the basis of the hydrophilicity and hydrophobicity of the grafted Al surface, the switching occurring with a change in temperature.
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