Active control of interfacial properties

Recent studies aimed at establishing principles for active control of the p hysicochemical properties of interfaces have made substantial progress towa rds demonstrating spatial and temporal control of interfacial properties of both liquids and solids. Light-active and redox-active surfactants have be en shown to permit large(> 20 mN/m) and reversible changes in the surface t ensions of liquids on time-scales of seconds. These changes can be directed to localized regions of liquids with sub-millimeter spatial resolution, th us providing new means to create controlled gradients in surfactant-based p roperties of liquids (e.g. gradients in surface tension). Progress has also been reported in the electrowetting of liquids on the surfaces of solids, although it is still not possible to use an external electric field to caus e an aqueous solution to wet a hydrophobic surface. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.