Ramin Golestanian FRS

Abstract:

Spontaneous symmetry breaking in colloidal systems increasingly fascinates scientists due to the promise of new, emergent functionalities and unique properties. Here, we introduce a chemically active system where symmetry is spontaneously broken in the particle motion rather than being encoded into their structure. Using symmetric MnO2 microrods without any apparent structural features or chirality, we observe the emergence of a combination of rotational and translational motion near a substrate that resembles rolling. This motion is further enhanced if a chemically active substrate is present. To explain the occurrence of highly active motion, we investigate the flows that lead to propulsion. The results are rationalized by a theoretical model showing spontaneous symmetry breaking with a bifurcation above a critical Péclet number. To identify the chemical reactions leading to the occurrence of flow, we confirm the species involved using electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). Overall, the results demonstrate the potential of spontaneous symmetry breaking to accomplish active rolling motion of symmetric chemically active microparticles.

Abstract:

Replication through cell division is one of the fundamental processes of life and a major driver of dynamics in systems ranging from bacterial colonies to embryogenesis, tissues and tumors. While regulation also shapes self-organization, many biologically relevant behaviors arise from a limited number of physical ingredients, and particle-based models have become a popular platform to investigate these emergent dynamics. However, incorporating division into such models often produces aberrant mechanical fluctuations that hinder meaningful analysis. Here, we introduce a minimal model ensuring mechanical consistency during cell division. Cells consist of two nodes, overlapping disks which separate during division, forming transient dumbbell shapes. Internal degrees of freedom, cell-cell interactions and equations of motion guarantee force continuity at all times, including during division, both for the dividing cell and its interaction partners, while allowing arbitrary anisotropic mobilities. As a benchmark, we also translate an established model of proliferating spherocylinders with similar dynamics into our theoretical framework. Numerical simulations demonstrate force continuity of the new disk cell model, quantify the improvements, and show agreement in terms of collective behaviors such as alignment and orientational order. We also demonstrate force extraction and a Voronoi-based interpretation in a confluent-tissue context—with a three-dimensional generalization in embryonic-like confinement. A reference implementation of the model in two and three dimensions is freely available as a Julia package based on InPartS.jl. Our model provides a framework for analyzing mechanical observables such as velocities and stresses, and can be readily extended with additional biological features.

Abstract:

We study the behavior of catalytically active droplets in multicomponent conserved mixtures affected by noise. Working in the thin interface limit, we analytically determine the state diagram of the system, characterized by multiple dynamical regimes, and verify our findings using numerical simulations. In particular, we show the emergence of a nonreciprocal, chemically mediated interaction between the droplets, which leads to the formation of (meta)stable clusters of droplets of different species. We find that the clusters can display self-propulsion in a large part of the parameter space, including regions where the nonreciprocal interactions between the droplets are purely attractive. This surprising feature arises from the nonlocal nature of the chemical interactions, and points to locality violations as a general mechanism for energy dissipation and emergence of out-of-equilibrium steady states in active matter.

Abstract:

We present the phenomenology of the one-dimensional nonreciprocal Cahn-Hilliard model for varying nonreciprocity $\left(\alpha \right)$ and different boundary conditions. At small $\alpha$ , a perturbed uniform state evolves to a defect-laden configuration that lacks global polar order. Defects are the sources and sinks of traveling waves. For a given $\alpha$ , defects with a unique wave number that increases monotonically with $\alpha$ are selected. A critical threshold ${\alpha }_c$ marks the onset of a transition to states with finite global polar order. For periodic boundary conditions, above ${\alpha }_c$ , the system shows traveling waves that are completely ordered. In contrast, traveling waves are incompatible with the Neumann and Dirichlet boundary conditions. Instead, for $\alpha \gtrsim {\alpha }_c$ , we find fluctuating domains that show intermittent polar order, and at large $\alpha$ , the system partitions into two domains with opposite polar order.

Abstract:

We investigate the self-diffusiophoretic motion of a catalytically active spherical particle confined within a wedge-shaped domain. Using the Fourier-Kontorovich-Lebedev transform, we solve the Laplace equation for the concentration field in the diffusion-dominated regime. The method of images is employed to obtain the first and second reflections of the concentration field, accounting for both monopole and dipole contributions of the particle's surface activity. Based on these results, we derive leading-order expressions for the self-induced phoretic velocity in the far-field limit and examine how it varies with the wedge opening angle and the particle's position within the domain. We focus on the contributions to the phoretic velocities arising from phoretic interactions, without accounting for hydrodynamic effects. Our findings reveal that the wedge geometry significantly affects both the magnitude and direction of particle motion. Our study provides a systematic framework for calculating the contributions to the phoretic velocity arising from concentration disturbances near corners, with implications for microfluidic design and control of autophoretic particles in confined geometries.