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We report on photoisomerization kinetics of azobenzene containing surfactants in aqueous solution. The surfactant molecule consists of a positively charged trimethylammonium bromide head group, a hydrophobic spacer connecting via 6 to 10 CH2 groups to the azobenzene unit, and the hydrophobic tail of 1 and 3CH2 groups. Under exposure to light, the azobenzene photoisomerizes from more stable trans- to metastable cis-state, which can be switched back either thermally in dark or by illumination with light of a longer wavelength. The surfactant isomerization is described by a kinetic model of a pseudo first order reaction approaching equilibrium, where the intensity controls the rate of isomerization until the equilibrated state. The rate constants of the trans-cis and cis-trans photoisomerization are calculated as a function of several parameters such as wavelength and intensity of light, the surfactant concentration, and the length of the hydrophobic tail. The thermal relaxation rate from cis- to trans-state is studied as well. The surfactant isomerization shows a different kinetic below and above the critical micellar concentration of the trans isomer due to steric hindrance within the densely packed micelle but does not depend on the spacer length. [ABSTRACT FROM AUTHOR]

We demonstrate in situ recorded motion of nano-objects adsorbed on a photosensitive polymer film. The motion is induced by a mass transport of the underlying photoresponsive polymer material occurring during irradiation with interference pattern. The polymer film contains azobenzene molecules that undergo reversible photoisomerization reaction from trans- to cis-conformation. Through a multi-scale chain of physico-chemical processes, this finally results in the macro-deformations of the film due to the changing elastic properties of polymer. The topographical deformation of the polymer surface is sensitive to a local distribution of the electrical field vector that allows for the generation of dynamic changes in the surface topography during irradiation with different light interference patterns. Polymer film deformation together with the motion of the adsorbed nano-particles are recorded using a homemade set-up combining an optical part for the generation of interference patterns and an atomic force microscope for acquiring the surface deformation. The particles undergo either translational or rotational motion. The direction of particle motion is towards the topography minima and opposite to the mass transport within the polymer film. The ability to relocate particles by photo-induced dynamic topography fluctuation offers a way for a non-contact simultaneous manipulation of a large number of adsorbed particles just in air at ambient conditions. [ABSTRACT FROM AUTHOR]