Quasi-1D sedimentation of Brownian particles along optical line traps

Abstract

We studied the Brownian motion of single 1 and 3 µm particles moving along optical line traps oriented in the gravity axis. The finite linear traps are formed using holographic optical tweezers which redistributes the incoming light beam into an elongated super Gaussian profile. Direct observation of the particle’s motion along the gravity axis allowed us to distinguish the non-equilibrium sedimentation of the particle, when the average velocity is 〈uz〉≠0, followed by equilibrium sedimentation when 〈uz〉=0 and the particle establishes the so-called gravitational length. We found that the bottom edge of the linear trap functions as a virtual “floor”, and subsequent Boltzmann distribution analysis of the equilibrium particle trajectories allows the estimation of the particle’s mass in the hundreds of femtograms range. In parallel, we also induced the physisorption of a macromolecule (hyaluronic acid) onto polystyrene microparticle in solution, detecting an increment of the particle’ mass of around 2 picograms. This suggests that the present technique can be used in surface chemistry, as it detects analyte absorption in solutions containing colloids by estimating the mass of both, the colloidal particle and the layer formed around it. On the other hand, we also found that when the longitudinal confinement established by the optical trap length is smaller than twice the particle gravitational length, particles remain suspended indefinitely even in the presence of a density mismatch with the medium, i.e., the notion of sedimentation disappears. Finally, we observed that the optical forces along the linear trap do not alter the Brownian motion of the particles significantly but can exert an extra pushing force on the smallest particles (1 µm) considered.