Chemotaxis and autochemotaxis of self-propelling droplet swimmers

Abstract

Chemotaxis and autochemotaxis play an important role in many essential biological processes. We present a self-propelling artificial swimmer system that exhibits chemotaxis as well as negative autochemotaxis. Oil droplets in an aqueous surfactant solution are driven by interfacial Marangoni flows induced by micellar solubilization of the oil phase. We demonstrate that chemotaxis along micellar surfactant gradients can guide these swimmers through a microfluidic maze. Similarly, a depletion of empty micelles in the wake of a droplet swimmer causes negative autochemotaxis and thereby trail avoidance. We studied autochemotaxis quantitatively in a microfluidic device of bifurcating channels: Branch choices of consecutive swimmers are anticorrelated, an effect decaying over time due to trail dispersion. We modeled this process by a simple one-dimensional diffusion process and stochastic Langevin dynamics. Our results are consistent with a linear surfactant gradient force and diffusion constants appropriate for micellar diffusion and provide a measure of autochemotactic feedback strength vs. stochastic forces. This assay is readily adaptable for quantitative studies of both artificial and biological autochemotactic systems.artificial swimmers | chemotaxis | autochemotaxis | microfluidics L ocomotion of living bacteria or cells can be random or oriented. Oriented motion comprises the various "taxis" strategies by which bacteria or cells react to changes in their environment (1). Among these, chemotaxis is one of the best-studied examples (2, 3): Cells and microorganisms are able to sense certain chemicals (chemoattractants or chemorepellents) and move toward or away from them. This is an essential function in many biological processes, e.g., wound healing, fertilization, pathogenic species invading a host, or colonization dynamics (4, 5). When the chemoattractant or chemorepellent is produced by the microorganisms themselves, the system exhibits positive or negative autochemotaxis. Thus, chemotaxis provides a mechanism of interindividual communication. Modeling such communication strategies is key to understanding the collective behavior of microorganisms (6-8) as well as flocks of animals like fire ants (9, 10).To model the swimming motion of microorganisms, various self-propelling artificial swimmer systems have been developed based on different mechanisms. Generally, there are two classes of swimmers: systems driven by and aligning with external fields (11-14), including chemotactic gradients, and selfpropelled swimmers, which move autonomously in homogeneous environments (15-21). Many autonomous swimmers additionally react to external fields, e.g., phototactic gradients (22).Biological autochemotactic systems exhibit very complex behaviors (23,24), where physical effects are intermingling with effects from various bioprocesses such as cell migration, metabolism, and division. To untangle these effects, there have been some design proposals for artificial systems, such as in ref.25, and simulations on the dynamics o...

Evolution of Self‐Propelled Objects: From the Viewpoint of Nonlinear Science

“…[170] Here, we will give some examples of artificial self-propelled objects that could "sense" their environmental physicochemical conditions. [175][176][177][178][179][180] As elf-propelled dropletd riven by the Marangonif low (section 2.2.3) can also response to an external gradiento fp H, [177] rare earth metal concentration, [178,179] surfactant concentration, [180] or light intensity. The object can spontaneously move even though its environment is homogeneous, in which its movingd irection is random.…”

Chemically Propelled Motors Navigate Chemical Patterns

“…Both single motor motion and the collective motion of small ensembles of motors are considered. Such motors are able to respond to gradients in fuel or product molecules, and show various types of dynamic behavior, for example, the chemotactic behavior seen in systems of self‐propelled swimmers 43, 44, 45, 46, 47. Therefore, chemical patterns that involve gradients in these species concentrations can influence the motions of motors.…”

Spontaneous membrane formation and self-encapsulation of active rods in an inhomogeneous motility field

“…One feasible realization consists of colloidal Janus rods driven by light [49,50], which can be exposed to almost arbitrary motility landscapes [13]. Similar possibilities exist for self-propelled droplets [11] or modular microswimmers steered by an electrolyte gradient [56]. Moreover, rod-like bacteria at high concentrations [37,53] may serve as another model system to observe smectic ordering in motility landscapes.…”

“…A relatively new avenue of research focuses on inhomogeneous motility fields, in which the particle self-propagation speed depends on the spatial coordinate. This is frequently encountered in actual biological or artificial systems where the swimming speed depends on an external stimulus, such as an externally imposed chemical [6][7][8][9][10][11], light [12,13] or flow field [14,15] of the solvent. Both linear gradients in motility [6,13,16] and stepwise constant motility fields [17] have been studied, but also more complicated motility ratchets [13,18] and even motility waves propagating in time [19][20][21][22].…”