Entropic interactions in suspensions of semiflexible rods: short-range effects of flexibility

Errors in Energy Landscapes Measured with Particle Tracking

Tracking Brownian particles is often employed to map the energy landscape they explore. Such measurements have been exploited to study many biological processes and interactions in soft materials. Yet video tracking is irremediably contaminated by localization errors originating from two imaging artifacts: the "static" errors come from signal noise, and the "dynamic" errors arise from the motion blur due to finite frame-acquisition time. We show that these errors result in systematic and nontrivial biases in the measured energy landscapes. We derive a relationship between the true and the measured potential that elucidates, among other aberrations, the presence of false double-well minima in the apparent potentials reported in recent studies. We further assess several canonical trapping and pair-interaction potentials by using our analytically derived results and Brownian dynamics simulations. In particular, we show that the apparent spring stiffness of harmonic potentials (such as optical traps) is increased by dynamic errors but decreased by static errors. Our formula allows for the development of efficient corrections schemes, and we also present in this work a provisional method for reconstructing true potentials from the measured ones.

Near wall dynamics of a spherical particle in crowded suspensions of colloidal rods - dynamic information from TIRM revisited

We performed total internal reflection microscopy (TIRM) experiments to determine the depletion potentials between probe spheres and a flat glass wall which are induced by long and thin, rod-shaped colloids (fd-virus), and probe the spatially resolved dynamics of the probe spheres. The dynamic information from the same raw TIRM intensity time traces is extracted in three different ways: by determining the spatially averaged diffusion constant of the probe sphere normal to the wall, by measuring the position dependence of the diffusion coefficient, and by measuring the particle's local drift velocity. Up to a concentration of about 6 times the overlap concentration of the rod-like colloids, the spatially averaged diffusion coefficient and the amplitude of the depletion potential are in surprisingly good agreement with theoretical predictions in which mutual interactions between the rods are neglected, that is, where the concentration is less than the overlap concentration. On increasing the depletant content even further, however, both the static and the averaged dynamic quantities begin to deviate from such theoretical predictions. In particular we find large deviations from the prediction by Mao, Cates, and Lekkerkerker [J. Chem. Phys., 1997, 106, 3721] based on the third order virial expansion for the rod concentration. It is shown that there are significant inaccuracies in TIRM measurements of diffusion coefficients due to the limited time range in which the mean squared displacements vary linearly in time, whereas mean displacements give much more accurate information concerning the probe sphere dynamics.

Superwettability Strategy: 1D Assembly of Binary Nanoparticles as Gas Sensors

Binary 1D nanowires consisting of both SnO₂ nanoparticles and Au nanorods are fabricated through a "substrate-particle solution template" assembling method, which shows highly enhanced gas sensitivity toward acetone under ambient conditions.

Tunable depletion potentials driven by shape variation of surfactant micelles

Depletion interaction potentials between micron-sized colloidal particles are induced by nanometer-scale surfactant micelles composed of hexaethylene glycol monododecyl ether (C_{12}E_{6}), and they are measured by video microscopy. The strength and range of the depletion interaction is revealed to arise from variations in shape anisotropy of the surfactant micelles. This shape anisotropy increases with increasing sample temperature. By fitting the colloidal interaction potentials to theoretical models, we extract micelle length and shape anisotropy as a function of temperature. This work introduces shape anisotropy tuning as a means to control interparticle interactions in colloidal suspensions, and it shows how the interparticle depletion potentials of micron-scale objects can be employed to probe the shape and size of surrounding macromolecules at the nanoscale.

Sphere to rod transitions in self assembled systems probed using direct force measurement

The influence of nanoparticle shape, in particular the sphere to rod transition, on surface forces and consequently the properties of colloidal fluids is an interesting but not well investigated phenomenon. Here, the surface force behaviour of concentrated surfactant solutions containing cetyltrimethylammonium bromide and sodium salicylate with micelle shapes varying from slightly prolate to high aspect ratio rods was measured. Atomic force microscopy (AFM) with both rigid particle and soft droplet probes was used with comparisons and analysis made using the Chan-Dagastine-White model. It is observed that small changes to the micelle shape result in no discernable differences to the surface force behaviour, however, once the micelles are elongated significantly, the long range forces adjust in nature from oscillatory to that of a single attractive force well. This highlights the importance that nanocolloid shape has on the behaviour and properties of emulsions and other colloidal fluids, specifically for emulsion flocculation and handling in systems of rod and worm like micelles.

Interaction potential and near wall dynamics of spherical colloids in suspensions of rod-like fd-virus

Averaged diffusivities of spherical colloids in solutions of rod-like particles, i.e. fd-virus, were measured with EWDLS and TIRM. While the experimentally observed near wall dynamics of the spheres in the absence of fd are well described by standard hydrodynamic theories, there are significant deviations at finite fd concentrations. Both experimental methods yield data which are significantly smaller than the theoretical predictions.

Unexpected slow near wall dynamics of spherical colloids in a suspension of rods

In this paper, we will show the influence of an additional rodlike component, that is, fd-virus, on the diffusion of spherical polystyrene colloids close to a wall. The sphere diffusivity normal to the wall, D perpendicular, is strongly affected by the presence of the rods, while the effect on the parallel diffusivity, D||, is less pronounced except in the immediate vicinity of the wall. We show that this observation cannot be explained by describing the effect of the rods as a simple mean field depletion potential alone.

Depletion interaction mediated by polydisperse rods

The interaction between a colloidal hard sphere of radius R and a wall or between two spheres in a dilute suspension of infinitely thin rods of length L is calculated numerically. The method allows the study of depletion potentials for any value of LR and, consequently, the influence of rod length polydispersity can be investigated. It was observed that both the depth and the range of the potential increase drastically if the relative standard deviation sigma of the length distribution is larger than 0.25, while the potential is virtually indistinguishable from that caused by monodisperse rods, if sigma < or similar to 0.1.

Effective potential between two spheres in a suspension of adhesive rods

Analytic treatment and Monte Carlo simulations are used to study the effective potential between two spheres in a suspension of rods whose ends can adhere to the surface of the spheres. When only one end of each rod is adhesive the effective potential changes from being attractive to repulsive with enhancing the adherence, but when both ends of each rod are adhesive the effective potential is not a monotonic function of the distance between the two spheres for strong adherence. As the adhesive strength is fixed, the range of the effective potential will increase with increasing the length of the rods. When the adhesive range is much smaller than the diameter of the spheres, its influence on single end adhesion is approximately linear and on two end adhesion is about quadratic. Our results are qualitatively consistent with a recent experimental work.

Depletion potentials induced by charged colloidal rods

We present direct depletion potential measurements for a single colloidal sphere close to a wall in suspensions of charged colloidal rods. In contrast to earlier studies of purely entropic systems (Helden et al. Phys. Rev. Lett. 2003, 90, 048301), here electrostatic interactions are important. These enhance the depletion attraction and lead to repulsive parts in the interaction potentials, indicating correlation effects between the rods.

Entropic interactions on a colloidal sphere near the edge of a terrace

We calculated the entropic interactions between a colloidal sphere and the hard edge of a terrace by means of Monte Carlo simulation with the acceptance ratio method. Comparison with the Asakura-Oosawa approximation indicates that the AO approximation is more accurate in this model than other known models. Our simulation results are also in qualitative agreement with the experimental results obtained by A.D. Dinsmore et al.

Direct measurement of entropic forces induced by rigid rods

We present the first direct depletion potential measurements for a single colloidal sphere close to a wall in a suspension of rigid colloidal rods. Since all particle interactions are as good as hard-core-like, the depletion potentials are of entirely entropic origin. We developed a density functional theory approach that accurately accounts for this experimental situation. The depletion potentials calculated for different rod number densities are in very good quantitative agreement with the experimental results.