Interacting with Active Particles

Physics has been extremely successful in describing and predicting the behaviors of different kinds of materials. Biological and living materials pose challenges due to their complexity and dynamics. Therein, one frontier of research is so called active particles, which refers to self-propulsive objects that can interact with each other and with their environment leading to complex and important behaviors. Examples of active particles include biological ones, such as flocking birds, swarming microbes and schooling fish, with fascinating collective states. In addition, synthetic objects that mimic the behavior of these biological active particles can be nowadays prepared using various physical and chemical techniques. While many of these systems are easy to observe – birds and the flocks with binoculars or with a naked eye, bacteria with a simple optical microscope, and so on, it is still very difficult to interact with these, often small and fast, active objects to study them beyond simple observation. Our understanding of the behavior of the active particles, both at single particle and collective levels, is still very limited, partly because of lack of ideal experimental tools to interact with them.

History has shown that scientific research on materials of various kinds is extremely useful for the society. Concrete examples of this can be seen everywhere around us: semiconductors in computers and mobile devices, advanced alloys in transportation systems, increasingly biodegradable plastics in consumer products and packaging, liquid crystals in television panels, and so on. Active particles and materials studied in this project are among the new frontiers of materials research. The results of this project will give scientists new tools to study these complex systems, leading to improved understanding of the behavior of micro-organisms as well as synthetic active particles that are envisioned to be functional components in next-generation technological devices.

The overall objective of this project is to develop new tools for interacting with active particles in order to study their behavior beyond what is possible currently. The new tools will be based on utilization of magnetic fields and forces, as those are expected to be compatible with both biological and synthetic active particles.

In the first half of this highly interdisciplinary and experimental project, we have developed several new magnetic techniques for interacting with active particles using magnetic fields. We have also developed deeper understanding of the model active particles used in this project, which include C. reinhardtii microalgae, E. coli bacteria, and electrohydrodynamically driven Quincke rollers, which are among the important model systems in the field of active matter. We have carried out synthesis of various novel magnetic particles and magnetic fluids (ferrofluids) that will be used to control the aforementioned active particles.

Significant progress beyond the state of the art has been made already in the first half of the project. Specifically, we have discovered anomalous charging of iron oxide nanoparticles in nonpolar solvents that are used to study the model active particles (Quincke rollers), which enables novel magnetic liquids with voltage-controlled magnetic response of potential technological importance. We have also discovered various new phenomena in electrohydrodynamically driven liquids, including active liquid droplets, and novel biocompatible ferrofluids. We have also discovered magnetically controlled behavior and phase transitions in various active matter systems. Detailed characterization and study of these is of the main focus in the second half of the project.

Periodic Reporting for period 3 - InterActive (Interacting with Active Particles)

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