  Chemotaxis and auto-chemotaxis play an important role in many essential
biological processes. We present a self-propelling artificial swimmer system
which exhibits chemotaxis as well as negative auto-chemotaxis. 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
have 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 have
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 versus
stochastic forces.
