Periodic Reporting for period 1 - ActiDoC (Active Doping in Colloids)
Reporting period: 2015-04-01 to 2017-03-31
A second project, concerned bacteria moving through a passive near-critical fluid mixture. For this study, a mixture of water and surfactant was found, in which bacteria are able to swim. The applicant examined the effect of the bacteria on the criticality of the fluid. The bacteria were found to locally influence the critical behaviour of the fluid, which manifested as a trail behind the swimming bacteria. The trail was found to originate from a preferential interaction of the bacteria with one of the phases of the fluid.
The candidate also examined a mixture of bacteria and cornstarch particles, under the influence of gravity. The cornstarch particles were found to be strongly influenced by the bacteria, in such a way that they would move much more actively than spheres of the same size. The sedimenting particles were found to increase their sedimentation speed in mixtures of active bacteria. Conceptually, this has been attributed to local alignment of the bacteria with the direction of sedimentation, thus locally reducing particle drag.
Additional work involved the examination and development of other model systems, such as self-propelled oil droplets on an air water interface, as well droplets of kinesin/microtubules. For these, the ongoing work involves the determination of the physical reasons behind their propulsion, as well as self-interactions and interactions with walls. Additional experimental work during this time includes an internal collaboration concerning the adhesion interactions of bacteria to a glass substrate (active bacteria – passive wall). The applicant also carried out simulations to complement the experimental work. He conducted Brownian Dynamics simulations to generate understanding and reach conclusions on the interactions between particles and bacteria. Moreover, he has additionally run Ising model simulations to promote the understanding for interactions between bacteria and the near critical fluid.
The work on bacteria swimming in critical fluids forays into a completely unexamined aspect of active-passive systems, and introduces a model system for examining such interactions. The work concludes that the bacteria locally modify the phase behaviour of a phase separating system to produce visible trails. Future work may use the inherent simplicity of a critically fluctuating system to start probing the effect of activity on interfaces of tuneable strength.
Generally, the work done on the examination of new active systems has produced a few likely candidates which may find use as model systems in the general active matter community, notably the self-propelled oil droplets on an air-water interface.
Besides the direct scientific impact, the action has been successful in allowing the fellow to successfully promote and expand his research into active matter physics. The Marie Curie fellowship has provided the fellow with enough time and freedom to learn a broad range of new tools, to acquire new writing skills and to broaden and deepen his scientific perspective and understanding of his future in science.