Energy landscape of colloidal dumbbells in a periodic distribution of light.

Using a ray tracing calculation, the energy landscape of dumbbells, made of spherical colloidal particles, interacting with a periodic distribution of light is calculated. As shown previously [E. Sarmiento-Gomez, J. A. Rivera-Moran and J. L. Aruaz-Lara, Soft Matter, 2018, 14, 3684], planar aggregates of spherical particles adopt discrete configurations in such light distribution. Here we focus on the case of colloidal dumbbells both symmetric and asymmetric from an experimental and theoretical point of view. It has been shown that the direct calculation using the ray tracing approximation is in excellent agreement with the experiment in spite of the fact that the particles size and the wavelength of the trapping light are comparable. We also corroborate, at least for the more simple case of a single particle in a parabolic light distribution, that the simple method used here provides the same results as the more complex and general Lorenz-Mie approach giving a more simple yet reliable method for the calculation of the energy landscape of colloidal aggregates in periodic light distributions.

Single particle states of colloidal particles in 2D periodic potentials.

Colloidal particles when subjected to a periodic array of potential wells are observed to adopt discrete stable configurations depending on the particle size/array wavelength ratio. Experimentally, the configuration states are determined for singlets, doublets and triplets of identical spheres in a periodic array of traps. The energy landscape of a single spherical particle is obtained by considering the refraction of the incident light as it passes throughout the particle. Then, the energy of a dumbbell is determined as the superposition of two singlets. The energy of a triplet is calculated as the superposition of a dumbbell and a single particle. As it is shown here, this direct method predicts accurately the stable particle configurations as observed in the experiments. The method can be generalized to obtain the potential energy of an n-particle aggregate, using as building blocks the energies of singlets and doublets.

Transition from diffusive to subdiffusive motion in colloidal liquids.

In this work we report experimental and theoretical results for the motion of single colloidal particles embedded in complex fluids with different interparticle interactions. The motion of particles is found to follow a similar behavior for the different systems. In particular, the transition from the short-time diffusive motion to the subdiffusive intermediate-time motion is found to occur when the square root of its mean squared displacement is in the order of 1 tenth of the neighbors' interparticle distance, thus following a quantitative criterion similar to Lindemann's criterion for melting.

Brownian motion of optically anisotropic spherical particles in polymeric suspensions.

We studied the rotational and translational diffusion of optically anisotropic liquid crystal particles embedded in semidiluted polymer solutions of Poly-Ethylene-Oxide (PEO) at different concentrations and different molecular weights. The polymer radius of gyration was chosen to be similar to the size of the probe particles and the polymer concentrations used are just above the crossover concentration. Thus, the system consists of solid probe particles moving in a sea of overlapping particles of similar size. We found that the behavior of both particle dynamics, rotational and translational, is similar in the range of concentrations considered here. In both cases, two linear diffusive regimes are observed, separated by a subdiffusive time interval. The spatial scale at which this intermediate regime appears shows a dependence on both the polymer concentration and molecular weight, and has a value similar to the thickness of the polymer-depleted layer usually found in this kind of systems. Additionally, we observe that the colloidal dynamic scales with the overlapping degree of the polymer particles.

Rheology and DWS microrheology of concentrated suspensions of the semiflexible filamentous fd virus.

Microrheology measurements were performed on suspensions of bacteriophage fd with diffusive wave spectroscopy in the concentrated regime, at different values of ionic strength. Viscosity vs. shear rate was also measured, and the effect of bacteriophage concentration and salt addition on shear thinning was determined, as well as on the peaks in the viscosity vs. shear curves corresponding to a transition from tumbling to wagging flow. The influence of concentration and salt addition on the mean square displacement of microspheres embedded in the suspensions was determined, as well as on their viscoelastic moduli up to high angular frequencies. Our results were compared with another microrheology technique previously reported where the power spectral density of thermal fluctuations of embedded micron-sized particles was evaluated. Although both results in general agree, the diffusive wave spectroscopy results are much less noisy and can reach larger frequencies. A comparison was made between measured and calculated shear modulus. Calculations were made employing the theory for highly entangled isotropic solutions of semiflexible polymers using a tube model, where various ways of calculating the needed parameters were used. Although some features are captured by the model, it is far from the experimental results mainly at high frequencies.