Light-controlled transient barriers

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In agreement with the overarching goal of the Research Unit TRANSIEVES we will design a light-controlled transient sieve for particles. Light-controlled transient sieves are expected to be switched on and off quite quickly and easily adapted to a desired purpose by simply changing the light intensity profile. The overarching goal of the subproject A2 is, in collaboration with B1, to design, realize and optimize a transient sieve based on light sensitive microswimmers in a transient light gradient. In this subproject the swimmers are thermophoretic Janus particles with a gold cap which converts light energy into heat. They are prepared and partially characterized together with subproject A5. The heat at the gold cap leads to a temperature gradient, which leads to an osmotic flow of the surrounding solvent along the surface of the Janus particle and a movement of the Janus particle opposite to the flow (momentum conservation). We plan to use a spatiotemporally modulated light field to create a transient sieve which can be easily opened and closed in a controlled and rapid manner. The particles can pass through or will be blocked by the sieve, depending on their properties such as size, surface morphology or charge. One of the main questions is how the dynamics of the self-propelled Janus particles relate to the dynamics of the temporally modulated light field to achieve an efficient sieve performance. Therefore, first, fundamentals of the propulsion of thermophoretic swimmers in a static light gradient have to be explored. For example, it needs to be clarified which particle parameters are responsible for a pronounced difference in velocity and direction of the particles in the light gradient. Experimental parameters such as light gradient and width of the gradient, as well as a suitable temporal modulation of the light field will be adjusted according to the results of subproject B1 to test the theory. Therefore, our existing set-up for tracking particles will be upgraded with a combination of an electrooptical modulator and a rotating mirror allowing to tailor the light field. The hypothesis is that the characteristic opening time of the sieve for optimal sieving performance is related to the diffusional rotation time of the Janus particles. Details will be analysed in B4. Furthermore, the outcome of subproject B1 will be used to create design rules for an efficient performance of the transient sieve. In a later period of the subproject, a microgel will be attached to the Janus particle as cargo, which can have different functions, such as changing the fluid dynamics around the particle, and/or acting as a stimuli sensitive container for smaller molecules, which can be carried inside the microgel across the barrier and released behind the transient barrier.