Brownian fluctuations, hydrodynamics and elasticity of a microhelix

We use optical tweezers to study the mechanic and hydrodynamic properties of micro-fabricated helices suspended in a fluid. In rigid helices we track Brownian fluctuations around mean values with a high precision and over a long observation time. Through the statistical analysis of fluctuations in translational and rotational coordinates we recover the full mobility matrix of the micro-helix including the off diagonal terms related with roto-translational coupling. Exploiting the high degree of spatial control provided by optical trapping, we can systematically study the effect of a nearby wall on the roto-translational coupling, and conclude that a rotating helical propeller moves faster near a no-slip boundary. We also study the relaxation dynamics of deformable micro-helices stretched by optical traps. We find that hydrodynamic drag only weakly depends on elongation resulting in an exponential relaxation to equilibrium.