Séminaire

Mardi 31 Janvier 2017 à 11h00.

Light-driven actuation of interfacial swimmers

''Light is a convenient way to remotely control the locomotion of small objects. Combined with the Marangoni effect, I will show that light-induced surface flows can produce powerful propulsion forces. Two experimental realizations are described and theoretically discussed. In the first example, a micro-sphere adsorbed at the water-air interface is heated up by a laser beam. Above a critical laser power, the particle escape the on-axis configuration and performs circular orbits around the axis. Velocities of several 100 micrometer/second are observed with just a few milliwatts of laser power [1]. Next I will discuss the transport of liquid marbles floating at the water-air interface. The water solution contains photosensitive surfactants, so that irradiation by UV light generates a surface-tension gradient. Strikingly, it is observed that, below a critical liquid thickness, the particles move in the direction opposite to the Marangoni flow [2]. We experimentally and theoretically demonstrate that this anti-Marangoni motion is driven by the flow-induced deformation of the liquid surface, which gravity-propels the non-wetting marble against the surface flow. [1] A. Girot, N. Danné, A. Würger, T. Bickel, F. Ren, J.-C. Loudet, and B. Pouligny, Langmuir 32, 2687 (2016) [2] N. Kavokine, M. Anyfantakis, M. Morel, S. Rudiuk, T. Bickel, and D. Baigl, Angew. Chem. Intl. Ed. 55, 11183 (2016)''