Skip to main content

Active Doping in Colloids

Periodic Reporting for period 1 - ActiDoC (Active Doping in Colloids)

Reporting period: 2015-04-01 to 2017-03-31

Soft Matter physics deals with the study of a broad range of materials including plastics, food and biological samples. In particular, looking at he motions of microscopic particles in a fluid, they exhibit random diffusive characteristics and is known as Brownian, a result of collisions of solvent molecules with the particles. Through their thermal motion, the fluctuating passive colloids are able to explore their surrounding space, which allows them the possibility of self-assembling into larger structures. A novel approach to soft matter materials comes through the introduction of active matter. Instead of solely relying on thermal fluctuations for motion, active particles have the ability to convert stored energy into self-propelled motion. Such active propulsion can originate from biological or physical or chemical processes. Self-propelled systems range from bacteria and algae to synthetic particles. Particularly, the physics of active and passive mixtures is far from understood. The general objective of this project has been to address the dynamical and structural effects of active motions on passive softmatter systems, mainly dealing with the effect of doping passive complex systems with small amounts of active constituents, studied using experiments and simulations. The study of such systems may lead to new ways towards the 'smart' processing and creation of materials or even create micro-devices for medical applications. We have examined a few model active and passive systems and have concluded that each studied system provides a unique perspective into the influence of activity on passive matter. The details of the activity and particular interactions for each pared system, provides for new problems to solve and understand, showing that general rules for these interactions are not yet apparent and that continued research on such model systems should be done to bring a more generalized scientific understanding into focus.
The applicant, examined in detail the interactions between particles and bacteria on a 2D surface. By using simultaneous imaging of both bacteria and particles, as well as tracking, he found two distinct active time scales for the particles. The bacteria and particle positions were correlated and the swimming bacteria were found to impart activity to the particles, with the assistance of simulations, the two time scales were found to be related to the modes of motion of the bacteria.
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.
Our work on particles interacting with bacteria has shown that close range interactions are a strong component to the activity transferred between active and passive constituents of a similar size. The bulk of current work cites hydrodynamic interactions as being most important, however, we show that this is not the case. This will allow for more accurate models to be made on the interactions between active and passive systems.
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.
Swimming E. Coli bacteria leaving trails in a near-critical fluid