Surfactants as a Tool for Coupling Light Into Hydrodynamic Flow: The Influence of Surfactant Hydrophobicity and Head Polarity

Abstract

Light-driven diffusioosmosis (LDDO) harnesses interfacial solute gradients to generate controllable fluid flows at solid-liquid interfaces, enabling non-mechanical manipulation of colloids at micro- and nanoscales. We study how molecular properties of azobenzene containing surfactants such as hydrophobic tail length, head-group charge, polarity, and valency govern the magnitude, direction, and onset of flow. Modulated light induces reversible trans-cis isomerization, generating dynamic surfactant and micellar gradients that drive particles along surfaces, enabling transport, collection, and patterning. Tail hydrophobicity primarily determines the onset and maximum velocity of transport. In contrast, head-group valency plays a minor role due to interfacial charge regulation, indicating that solute-gradient-driven effects dominate over simple electrostatic screening arguments. Head-group polarity strongly affects flow direction: cationic and anionic surfactants yield flows governed by cis-isomer gradients, whereas non-ionic surfactant shows reversed motion linked to trans-isomer aggregation and adsorption. Temperature further modifies flow in non-ionic systems by altering aggregation and surface coverage. These results demonstrate that LDDO is governed by coupled processes of micellization, adsorption, and photoisomerization, enabling rational control of colloidal transport by molecular design. Overall, LDDO provides a reconfigurable light-driven platform for directing interfacial flows and manipulating colloids, with relevance for microfluidics, soft matter, and active materials.