Living islands of driven two-dimensional magnetic colloids on the disordered substrate

Abstract

We investigate, by Langevin simulations, the depinning of driven two-dimensional magnetic colloids on a substrate with randomly distributed pinning centers. The magnetic colloids are modeled as particles interacting with each other through repulsive magnetic dipole and attractive Lennard-Jones potentials. We find living islands above the depinning when the attraction between colloidal particles dominates. The critical pinning force increases and the living islands disperse gradually with an increasing strength of the pinning centers. But with an increasing strength of the repulsion between colloidal particles, the critical pinning force increases first and then decreases, and the living islands appearing at the low repulsion strength disappear when the repulsion strength is increased above certain value at which the peak effect takes place. When the repulsion strength is further increased, moving smectic and moving crystal structures arise above the depinning where the repulsion between colloidal particles dominates. On increasing the attraction strength, we find that the critical pinning force, which remains unchanged basically at the low attraction strength, decreases and the living islands form gradually in the plastic flows. When the attraction strength is increased to be large enough, the critical pinning force becomes unchanged again and the living islands and phase separation between different islands occur manifestly. The thermal fluctuations at low temperatures compete with the pinning potential, conducive to the formation of living islands, but they smooth the pinning potential at high temperatures, leading to the rapid decrease in the critical pinning force and the dispersion of the living islands.

Friction of two-dimensional colloidal particles with magnetic dipole and Lennard–Jones interactions: A numerical study

“…The most striking behaviors include the formations of clusters with finite size or islands with clear boundaries (the so-called living islands) [20][21][22][23][24][25]. These structures are closely related to biological self-organization [17,[20][21][22][23][24][25]; in this context, we note that biological activity is also one of the remarkable characteristics of biotribological problems in relation to general tribological problems.…”

“…Active systems can actively absorb energy from the environment and overcome resistance (i.e., energy barrier) through energy storage [14], as in the case of bacterial colonies of fish and birds and tissues of cells; such systems are different from passive systems that acquire energy from the surrounding environment and subsequently restore energy to the environment. It is known that the abovementioned activity originates from weak attractive interactions between individuals in the systems [15][16][17][18][19][20], for, e.g., van der Waals interaction between colloidal particles. Such self-propelling interactions between individual members of active systems [21] drive the systems to non-equilibrium states, which leads the systems exhibiting a series of novel behaviors that cannot be observed under equilibrium [20][21][22][23][24][25].…”

“…It is known that the abovementioned activity originates from weak attractive interactions between individuals in the systems [15][16][17][18][19][20], for, e.g., van der Waals interaction between colloidal particles. Such self-propelling interactions between individual members of active systems [21] drive the systems to non-equilibrium states, which leads the systems exhibiting a series of novel behaviors that cannot be observed under equilibrium [20][21][22][23][24][25]. The most striking behaviors include the formations of clusters with finite size or islands with clear boundaries (the so-called living islands) [20][21][22][23][24][25].…”

“…Such self-propelling interactions between individual members of active systems [21] drive the systems to non-equilibrium states, which leads the systems exhibiting a series of novel behaviors that cannot be observed under equilibrium [20][21][22][23][24][25]. The most striking behaviors include the formations of clusters with finite size or islands with clear boundaries (the so-called living islands) [20][21][22][23][24][25]. These structures are closely related to biological self-organization [17,[20][21][22][23][24][25]; in this context, we note that biological activity is also one of the remarkable characteristics of biotribological problems in relation to general tribological problems.…”

“…The substrate is a realistic quenched one, which is different from an ordered substrate [27]. The active magnetized colloidal particles are modeled as interacting with each other via repulsive magnetic dipole and attractive Lennard-Jones potentials [20,28]. We attempt to determine the relationship between sliding friction and dynamic phases and phase transitions, which can provide a theoretical framework for the control of biological self-organization via utilization of the friction properties of mesoscopic colloidal systems.…”