Poly(N-isopropylacrylamide) grafting on aluminium to actively switch its surface drag in water

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 degrees and 65 degrees for (B) at temperatures of 25 and 40 degrees 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 degrees C in comparison to that at 22 degrees 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. (C) 2010 Society of Chemical Industry