Research

For a copy of my dissertation, click here.

Dynamical Processes in Dusty Plasmas

Bistable Switching

The course of my Phd studies was determined by a serendipitous event, in which I observed the recurrent melting and recrystallization of a layer of dusty plasma crystal. Dusty plasma is a state of matter consisting of microscopic particles immersed in weakly ionized gas (plasma) and they can be used as a model system to investigate emergent phenomena in complex systems. Our discovery revealed that complex systems that are made of monostable (can be only in one state) elements can manifest bistable switching.

This discovery led us on a two-fold path: one elucidating the origin of self-induced vertical oscillations, which provide energy for the transition, and another one investigating the mechanism behind the emergence of a “clock” that governs the temporal behavior of the system.

Self-induced Vertical Oscillations

Below certain plasma pressure, dust particles in plasma experience self-induced vertical oscillations. Since the initial observation of these oscillations in 1999, various mechanisms have been proposed to explain these oscillations: delayed charging, stochastic charge variations and inhomogeneous environment of plasma.

Currently, we are investigating the self-induced vertical oscillations using a custom made Langmuir probe to characterize the plasma environment and high-speed imaging of single particles. Our measurements show that the oscillation amplitude can reach a few centimeters, which is an order of magnitude larger than the amplitudes reported previously.

Switching Mechanism

We found that the switching behavior in our system is a consequence of the coupling between a noisy environment and structural heterogeneity. Using numerical simulations we found that for the switching to occur, the system should have some quenched disorder, which is achieved by the Gaussian distribution of particle sizes. If the particles are all the same size, then the system will oscillate like a rigid body and the melting transition will not occur.

Despite the fact that the switching is a non-equilibrium phenomenon, we have found that the linear properties of the system can predict the dynamical features of the dynamics. Namely, using dynamical matrix formalism, we investigated the normal modes of the system and found that the switching intensity is maximized for some intermediate fraction of localized modes. It is quenched disorder that gives rise to localized modes and noisy driving that excites them. Upon excitation, the localized modes transfer energy to other modes and the system melts.

This Youtube video summarizes our research findings.

Minimal Model

Bistable switching shares qualitative characteristics with intermittent turbulence in a pipe flow. As the laminar flow in a pipe is linearly stable even for infinitely large Re numbers, for the transition to turbulence to occur, the driving (pressure difference) and structural heterogeneity (from the roughness of the pipe walls) should be large enough. Inspired by recent work that treated turbulent and zonal flow energies as interacting as predator-prey system, we constructed a minimal model that treats the vertical and horizontal energies as prey and predator. Coupling between these two energies is controlled by the amount of structural disorder – in the absence of it, the energies do no interact at all.

The model recreates the phenomenon observed in the experiments and numerical simulations without considering any system-specific properties. If you would like to learn more about this model, you can do so here.