Fundamental Active Matter:
Active matter is the area of research that aims to identify the shared physical principles that govern the collective behavior of inherently out of equilibrium living systems. To reduce the complexity of living systems, simple model-systems with few variables, including Active Brownian Particle suspensions (ABPs), are used as alternatives. In ABPs, in addition to the thermal fluctuations and conserved interparticle interactions, each particle also undergoes a directed motion. I have an ongoing collaboration with Klotsa Lab (APS, UNC-CH), where we study the collective dynamics of ABPs, using particle simulations and coarse grain nonequilibrium theories.
Surface tension of soft active Brownian suspensions:
It is well-known that above a critical activity ABPs undergoes a motility-induced phase transition (MIPS), which results information of a densely packed cluster that is surrounded by a gas-like phase of active particles. The cluster maintains its mechanical integrity through the alignment of active particles at the interface between the dense and the gas phase; see the movie on the right!
In passive systems, the mechanical stability of droplets is achieved by surface tension. This raises the question: Can we define an equivalent of an effective surface tension in active systems that stabilizes the cluster after MIPS? Through a combination of particle simulations and coarse-grained theories we found that surface tension is nearly zero or the phase separated domains, irrespective of activity, softness, and area fraction. The results can be found in this paper.
ABPs mixtures with different activities:
We use Brownian Dynamics particle simulations to study the steady-state structure and mechanics of ABPs after MIPS in binary active systems of slow and fast ABPs. We find many novel features, including nmonotonic macroscopic properties, microphase separation, increased fluctuations, and severe avalanche events, that not present in ABPs with a single activity. Our combined theory and simulation results point to the following mechanism: MIPS begins with the nucleation of small clusters of fast particles. These clusters act as rigid boundaries on which slow and fast particles adsorb. The difference in activities leads to microphase separation of slow and fast particle domains at the interface with fast particles assembling at the inner layer closer to the bulk and the slow particles assembling at the outer layer of the interface closer to the gas phase. The microphase-separated domains at the interface lead to stronger fluctuations in the compression force which then turn leads to enhanced avalanche events. As more particles assemble around the existing interface, the cluster grows, and the phase separated slow and fast particles that were making up the interface at earlier times get integrated to the cluster. The process repeats with the re-adsorption of fast and slow particles from the gas onto the interface. See the video on the right!
Related Publications:
Lauersdorf, N., T. Kolb, M. Moradi, E. Nazockdast, and D. Klotsa, 2021. Phase behavior and surface tension of soft active Brownian particles. Soft Matter 17:6337–6351.
Lauersdorf, N. J., E. Nazockdast, and D. Klotsa, 2024. Using Activity to Compartmentalize Binary Mixtures. https://arxiv.org/abs/2407.07826.