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Microrheology of two-dimensional active colloidal crystals and glasses

Periodic Reporting for period 1 - MicACol (Microrheology of two-dimensional active colloidal crystals and glasses)

Reporting period: 2017-09-04 to 2019-09-03

Synthetic microswimmers are a new class of colloidal particles that have recently become hugely popular. In contrast to conventional colloids that only jiggle around in the fluid due to thermal fluctuations, artificial microswimmers (or active particles) are in fact able to convert energy from the environment into net motion. They are the man-made equivalent of living microorganisms such as bacteria, algae or prokaryotic cells. Furthermore, they are promising building-blocks for the fabrication of novel materials with dynamic properties. The last decade has witnessed a growing research aimed at understanding the physics of self-propulsion at the micro- and nanoscale. Non-equilibrium collective phenomena have been investigated in depth to highlight fundamental links to living swarms, schools and flocks. However, technical difficulties have prevented the research field from moving towards denser colloidal phases including crystalline and glassy structures. The main goal of this project was to fabricate colloidal crystals made of active particles and investigate their mechanical properties.
To achieve these objectives, we decided to assemble two-dimensional active crystals at a flat interface between two immiscible fluids. The reason for this choice lies in the fact that at the liquid-liquid interface colloidal particles can interact via long-ranged dipolar repulsive forces and therefore form crystalline phases at low packing fraction.

We first designed and characterized active colloids that are capable of swimming at the fluid-fluid interface. We used catalytic colloids that self-propel due to the decomposition of hydrogen peroxide across a Platinum cap. Later, we verified that such microswimmers can exhibit long-ranged repulsion when they interact with other particles that are also confided in the interfacial plane. In particular, suitable interactions are found when the active particles are orientated such that the Platinum cap and the uncoated hemisphere are wetted by the water sub-phase and the oil upper phase, respectively. We studied the dynamics (i.e. the active trajectories) of these particles inside a crystalline monolayer and focused our attention on one rheological property: the ability of active colloids to leave their equilibrium lattice positions. A direct comparison with colloids manipulated by an optical trap revealed that self-propelling colloids behave in a crystal as if they were driven by an effective force proportional to their free swimming velocity.
The project shows the first experimental realization of active crystals with repulsive interactions. We expect this model system to become useful for further fundamental and applied studies. Fundamental examples to which our fabrication method can be applied include polycrystalline structures, crystalline defects, colloidal glasses and hexatic phases. In contrast, foreseen applied studies include particle-stabilized emulsions and active interfaces. Of course, we are still far from the current dream in the field of synthetic active matter, i.e. the fabrication of 3D active materials. Nonetheless, we foresee that our results concerning the dynamical and mechanical properties of active crystals will remain valid for more advanced materials.
Active crystal made of Brownian and active colloids