High precision measurement of short-ranged colloidal interactions with digital holographic microscopy

Though short-ranged attractive interactions such as the depletion interaction are widely used in the field of colloidal self-assembly, direct characterization of the interaction potential is currently limited by the precision of measurement techniques. We show that short-ranged colloidal interactions can be measured optically at separation distances as small as a few nanometers. We do this by combining digital holographic microscopy and forward modeling, which allows us to precisely track pairs of interacting colloidal particles in three dimensions and at high frame rates. Through a Bayesian approach that accounts for uncertainties in the sizes and positions of the particles, we infer the pair potential from measurements of the fluctuations in separation distance. We validate the results by comparison with indirect measurements of the interaction.