Measuring colloidal interactions with confocal microscopy

Abstract

We use confocal laser scanning microscopy to measure interactions in colloidal suspensions. By inverting the radial distribution function, determined by tracking the particle coordinates, we obtain the effective interaction between the colloidal particles. Although this method can be applied to arbitrary colloidal interactions, here we demonstrate its efficacy with two well-known systems for which accurate theories are available: a colloid-polymer mixture and binary hard spheres. The high sensitivity of this method allows for the precise determination of complex interactions, as exemplified, for example, by the accurate resolution of the oscillatory effective potential of the binary hard sphere system. We argue that the method is particularly well suited for the determination of attractive forces.

Effective interactions between soft-repulsive colloids: Experiments, theory, and simulations

“…This results typically in a broadening of the peaks in g(r), combined with a significant reduction of the peak height and a shift of their positions. These artifacts have already been noticed in previous studies with hard sphere-like [30][31][32] and microgel colloids. 24 Unfortunately, the extent of these various contributions in quantitative CLSM data varies with the system considered, and depends for example on φ eff due to the concentration dependence of the particle mean square displacement r(t) 2 in relation to the scanning speed, but also on the degree of particle labeling, the particle size and the mismatch of refractive index, to name some of the more important factors only.…”

“…Minor but noticeable discrepancies between model (simulation) and experimental results have also been observed in previous studies on well-defined classical hard sphere systems 31,34 or hard sphere colloids with depletion interactions. 30 We feel that the most important point for claiming quantitative agreement is the ability to not only reproduce an individual g(r), but recover the volume fraction dependence of the main features of g(r). In our case, we clearly demonstrate that once we fix the strength of the Hertzian interaction parameter for one concentration, we can reproduce the experimental data within experimental accuracy over a fairly extended range of densities without any additional adjustable parameters.…”

“…30 In accordance with the poorer experimental resolution along the z-direction, we randomly displace one of the coordinate (e.g., z) of all particles in each configuration by a Gaussian noise taken from a distribution with standard deviation w centered around unity.…”

The role of quench rate in colloidal gels

“…Here, although full phase separation does not occur, a very considerable number of particles are able to reach local equilibrium and crystallise. We have based our discussion on a slightly polydisperse system as found in experiments [28,30]. This has two main consequences.…”

“…In the experimental system upon which we base our simulations, residual electrostatic interactions are screened [28,30] and are neglected here. We found good agreement with simulations using the Gelation results upon quenching, provided the quench rate is sufficient to avoid phase separation (shaded region).…”