8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P6.5Pumping-out photo-surfactants from an air-waterinterface using lightEloise Chevallier, 1 Alexandre Mamane, 1 Howard Stone, 2 Christophe Tribet, 3François Lequeux, 1 and Cécile Monteux 11 ESPCI-PPMD, 10 rue vauquelin, 75005, Paris, France2 Dept Mech Aerosp Engn, Princeton, Ud States of Am3 Pôle Chimie Biophysique UMR 8640, Paris, FranceIn this work, we investigate how a light stimulus can tune interfacial properties of an air-waterinterface with photo-switchable surfactants. This system is indeed very promising to control thestability of foams or emulsions. The azobenzene moiety in the apolar tail of the surfactant partlyswitches from a trans to a cis conformation under UV (and vice versa under blue light). Fromdynamic surface tension measurements and numerical simulations, we characterized the competitionbetween the two isomers: the cis isomers desorb 300 times faster than the trans. So, withoutany stimulation, the interface is quickly enriched with the trans, the more surface active isomer ofthe two. Besides, for an illuminated interface we modeled the competition between the adsorptionand the cis-trans photo-conversion. We evidenced that the light stimulus -both UV and blue- introducesat the interface some cis isomers that quickly desorb. So, the total surface excess decreasesthe stimulus induces a “pumping-out” of surfactant from the interface. As a result, we show thatthe intensity of the stimulus can simply tune the surface tension ans the velocity of Marangoniflows.5