Recently, we have discovered a peculiar phenomenon related to colloids adsorbed at a solid- liquid interface, and immersed into aqueous solution of photosensitive surfactant (photosoap). Upon illumination with UV light, the colloids are driven out of the irradiated area. Without the presence of the surfactant, the particle assembly does not change, ruling out heating effects or gradients in the intensity of the electromagnetic field as a source of driving. The presence of the surfactant leads to two simultaneous effects. The first involves the local modification of surfaces even without illumination. The most important and novel aspect to be discussed in the talk is, however, that the trans-cis photo-isomerization is also the basis for initiation of local hydrodynamic flows and corresponding forces on the colloids. The photosoap consists of a charged ammonium bromide head bound to a hydrophobic tail incorporating an azobenzene group. Azobenzene molecules undergo reversible trans-cis photo- isomerization when illuminated with either UV or visible light leading to substantial changes in physico-chemical properties e.g. trans-to-cis-isomerization decreases the hydrophobicity of our photosoap. We will provide a theoretical account of how the local liquid flows emerge. It will turn out that the phenomenon is best understood as light-driven diffusioosmosis. In this way, the surfactant becomes a photo-soap in a new sense: not only particle-surface interaction is reduced, but also control over the “rinsing” of contaminants (particles) can be gained.

Light driven diffusioosmosis: Manipulation of particle assembly

253rd National Meeting (2017)

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