Swimming to Stability: Structural and Dynamical Control via Active Doping

Abstract

External fields can decidedly alter the free energy landscape of soft materials and can be exploited as a powerful tool for the assembly of targeted nanostructures and colloidal materials. Here, we use computer simulations to demonstrate that nonequilibrium internal fields or forces -forces that are generated by driven components within a system -in the form of active particles can precisely modulate the dynamical free energy landscape of a model soft material, a colloidal gel. Embedding a small fraction of active particles within a gel can provide a unique pathway for the dynamically frustrated network to circumvent the kinetic barriers associated with reaching a lower free energy state through thermal fluctuations alone. Moreover, by carefully tuning the active particle properties (the propulsive swim force and persistence length) in comparison to those of the gel, the active particles may induce depletion-like forces between the constituent particles of the gel despite there being no geometric size asymmetry between the particles. These resulting forces can rapidly push the system toward disparate regions of phase space. Intriguingly, the state of the material can be altered by tuning macroscopic transport properties such as the solvent viscosity. Our findings highlight the potential wide-ranging structural and kinetic control facilitated by varying the dynamical properties of a remarkably small fraction of driven particles embedded in a host material.Advances in materials chemistry have opened new and exciting vistas for designing colloidal and nanoparticle building blocks, resulting in materials endowed with unique structures and accompanying properties. 1-4 Particle shape 5,6 and surface chemistry (e.g., decorating the particle surface with sticky "patches" 7,8 of organic or biological molecules 9-11 ) allow for exquisite control of the directionality of particle interactions, unlocking a multitude of structures with wide-ranging symmetries. The attractive particle interactions that are often used to stabilize these structures may also introduce a free energy landscape replete with barriers associated with particle rearrangement which can significantly delay or even preclude reaching the globally stable configuration. Indeed, the traditional pathways for homogeneous suspensions of particles to phase separate into dilute and dense phases, including spinodal decomposition and nucleation, can be exceedingly long and in practice can be the limiting factor in the experimental observation of ordered colloidal and nanoparticle-based materials. 12,13The classical system that demonstrates this issue of kinetic frustration is perhaps also the simplest: a colloidal suspension of spheres with short-range isotropic attractive interactions. While the equilibrium phase diagram of this system predicts fluid-solid coexistence for interaction strengths as little as a few times the thermal energy k B T , in practice the system is found to form a long-lived space-spanning network, a gel, for nearly the entire coexistence regio...

Reverse osmotic effect in active matter

“…Here, Pe 1,2 is the global Péclet number and it represents the relative significance of advective transport in region 1 compared to diffusive transport in region 2. By comparing (A1)-(A6) and (A7)-(A14), we observe an interesting difference between passive-like ABPs (U > 0 and Pe → 0) and passive Brownian particles (PBPs; U = 0 and so Pe = 0), which show the identical diffusion-dominated dynamics [31,32]. The number density changes only when there exists variation of swim speed.…”

Active binary mixtures of fast and slow hard spheres

“…Mixtures provide an additional degree of control for performing specific functions. For example, doping a monodisperse Brownian system with ac-tive particles can help anneal crystals and rapidly relax jammed states [26,27]. Many active living systems interact not only with passive Brownian particles and obstacles but also with other active species, e.g.…”

Activity-controlled annealing of colloidal monolayers

“…Opportunities in colloidal assembly were further stressed by recent numerical works that considered mixtures of active, autonomously-driven particles, in passive phases. They showed that the presence of active particles can favor the annealing of a passive polycrystal by melting its grain boundaries 25–27 or act as an internal field to control the structural and dynamical properties of a colloidal gel 28 . Notwithstanding, experimental realizations of mixtures of active particles in a passive phase are scarce and have not demonstrated control of the passive phase.…”