Chemo and phototactic nano/microbots

Abstract

One of the more interesting recent discoveries has been the ability to design nano/ microparticles which catalytically harness the chemical energy in their environments to move autonomously. These "nanomotors" can be directed by externally applied magnetic fields, or optical and chemical gradients. Our group has now developed two systems in which chemical secretions from the translating micro/nanomotors initiate long-range, collective interactions among the particles via self-diffusiophoresis. Herein, we discuss two different approaches to model the complex emergent behavior of these particles, the first being a qualitative probability-based model with wide applicability, and the second being a more quantitative Brownian dynamics simulation specific to the self-diffusiophoretic phenomenon.

Pseudochemotactic drifts of artificial microswimmers

“…For artificial microswimmers under the most common experimental conditions, D φ is only weakly affected by the ρ gradient, while v 0 (x) is reported to grow linearly with ρ and then saturate at higher ρ [9,[14][15][16]. The onset of "anti-Fick" cold-to-hot (pseudo-chemotactic) currents can thus be easily demonstrated.…”

“…At the channel ends, x = ±L/2, two porous membranes allow the chemical flow in and out, but prevent the JP from escaping into the reservoirs. We speculate, based on experimental observations [9,[14][15][16], that both v 0 and D φ (and therefore D s ) may depend on ρ(x) to some unspecified extent. We only assume that both v 0 (x) and D φ (x) are non-decreasing functions of the channel coor- …”

“…This might sound paradoxical, but we came to the conclusion that the injected JP is finally attracted toward the left (cold) exit, even if, immediately after injection, it may drift to either direction, depending on the x-dependence of the propulsion parameters v 0 and τ φ . For the most common case when the x-dependence of τ φ is much weaker than D s [9,[14][15][16], the injected particle points decidedly to the right (hot) exit. Reconciling these seemingly conflicting mechanisms is of paramount importance to control the chemotaxis of artificial microswimmers as opposed to bacterial chemotaxis [17,18].…”

Chemo-responsive, self-oscillating gels that undergo biomimetic communication

“…In this review, we describe examples of BZ gels that show some salient features of this mode of communication. We note that while researchers have developed a range of nano-and microscopic self-propelled particles, 28 few of these exhibit auto-chemotaxis, 29 and even fewer 30 use this mechanism to self-organize into large-scale structures. Research on synthetic auto-chemotaxing particles is still in its infancy, but recent advances 2,3 point to promising efforts in the creation of mechanically compliant materials that appear to communicate and effectively collaborate.…”

Active Brownian motion in a narrow channel

“…Active chiral motion has long been known in biology [20,19,21] and more recently observed in asymmetrically propelled micro-and nano-rods: A torque can be intrinsic to the propulsion mechanism, due to the presence of geometrical asymmetries in the particle fabrication, engineered or accidental (asymmetric JP's) [22,24,23], or externally applied, for instance, by laser irradiation [14] or hydrodynamic fields [25]. In the finite damping regime, the Lorentz force exerted by a magnetic field on a charged active Brownian particle also amounts to an external torque [26]; however, the effects of such magnetic torques vanishes in the overdamped limit [27].…”