colloids; force inference; external field; pair interaction
There are two types of methods to extract the forces acting on colloids: equilibrium- distribution and drift-diffusion methods [1,2,3]. While the former can only be used when the system is in thermodynamic equilibrium, drift-diffusion methods relying on displacement measurements do not require this condition. Alas, because of the solvent agitation, deterministic displacements caused by the forces are tangled with Brownian motion and when this thermal noise is heterogeneous, particles are subject to a noise-induced spurious force that adds another layer of complexity [4,5]. Recently, a novel method based on information and communication theory was introduced to infer force-fields and diffusion from high dimensional stochastic trajectories . Within this approach, we develop a framework for the analysis of trajectories of invidual colloids: we first validate our approach with molecular dynamics simulations and we then successfully apply it to three-dimensional trajectories of individual colloids measured using a modified version of 3D-phase contrast holographic microscopy . Within this framework we are able to infer the one-body and two-body interactions as well as the diffusion and spurious force in different solvents. Our method also allows to investigate the measurement error to optimize the acquisition protocol.
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