Gerard H. Wegdam

Salt Crystallization during Evaporation: Impact of Interfacial Properties

Salt damage in stone results in part from crystallization of salts during drying. We study the evaporation of aqueous salt solutions and the crystallization growth for sodium sulfate and sodium chloride in model situations: evaporating droplets and evaporation from square capillaries. The results show that the interfacial properties are of key importance for where and how the crystals form. The consequences for the different forms of salt crystallization observed in practice are discussed.

Multiple nonergodic disordered states in Laponite suspensions: A phase diagram

We study the time evolution of different Laponite suspensions from a low-viscosity ergodic state to a viscoelastic nonergodic state over a wide range of volume fractions and salt contents. We find that the evolution of nonergodicity parameter (Debye-Waller factor) splits into two branches for all the samples, which correspond to two distinct dynamically arrested states. At moderately high salt concentrations, on the other hand, a third nonergodic state appears that is different from the above two nonergodic states. Measurement of the conductivity of Laponite solutions in pure water shows that the contribution of counterions in the ionic strength is considerable and their role should be taken into account in interpretations of aging dynamics and the phase diagram. Based on these data and available data in the literature, we propose a (nonequilibrium) phase diagram for Laponite suspensions.

Long-range strain correlations in sheared colloidal glasses

Glasses behave as solids on experimental time scales due to their slow relaxation. Growing dynamic length scales due to cooperative motion of particles are believed to be central to this slow response. For quiescent glasses, however, the size of the cooperatively rearranging regions has never been observed to exceed a few particle diameters, and the observation of long-range correlations has remained elusive. Here, we provide direct experimental evidence of long-range correlations during the deformation of a dense colloidal glass. By imposing an external stress, we force structural rearrangements, and we identify long-range correlations in the fluctuations of microscopic strain and elucidate their scaling and spatial symmetry. The applied shear induces a transition from homogeneous to inhomogeneous flow at a critical shear rate, and we investigate the role of strain correlations in this transition.

Controlling colloidal phase transitions with critical Casimir forces

The critical Casimir force provides a thermodynamic analogue of the quantum mechanical Casimir force that arises from the confinement of electromagnetic field fluctuations. In its thermodynamic analogue, two surfaces immersed in a critical solvent mixture attract each other due to confinement of solvent concentration fluctuations. Here, we demonstrate the active assembly control of colloidal equilibrium phases using critical Casimir forces. We guide colloidal particles into analogues of molecular liquid and solid phases via exquisite control over their interactions. By measuring the critical Casimir pair potential directly from density fluctuations in the colloidal gas, we obtain insight into liquefaction at small scales. We apply the van der Waals model of molecular liquefaction and show that the colloidal gas-liquid condensation is accurately described by the van der Waals theory, even on the scale of a few particles. These results open up new possibilities in the active assembly control of micro and nanostructures.

Fluctuation-dissipation theorem in an aging colloidal glass

We provide a direct experimental test of the fluctuation-dissipation theorem (FDT) in an aging colloidal glass. The use of combined active and passive microrheology allows us to independently measure both the correlation and response functions in this nonequilibrium situation. Contrary to previous reports, we find no deviations from the FDT over several decades in frequency (1 Hz-10 kHz) and for all aging times. In addition, we find two distinct viscoelastic contributions in the aging glass, including a nearly elastic response at low frequencies that grows during aging.

Dynamics of dense, charge-stabilized suspensions of colloidal silica studied by correlation spectroscopy with coherent X-rays

The dynamics of concentrated, charge-stabilized colloidal silica suspensions was studied over a wide range of wave-vectors. The short-time diffusion coefficient, D(Q), was measured for concentrated suspensions up to their solidification points by photon correlation spectroscopy with coherent X-rays and compared to free particle diffusion D0, studied by Dynamic Light Scattering (DLS) in the dilute case. Small angle X-ray scattering (SAXS) was used to determine the static structure factor S(Q). D0/D(Q) peaks for Q values corresponding to the maximum of the static structure factor showing that the mostly likely density fluctuations decay the slowest. The data allow one to estimate the diffusion coefficient D(Q) in the Q 0 and Q limits. Thus, hydrodynamic functions can be derived free from any modeling of the static or dynamic properties. The effects of hydrodynamic interactions on the diffusion coefficient in charge-stabilized suspensions are presented for volume fractions 0.075 < < 0.28.

Acoustic band gaps in composites of solids and viscous liquids

Abstract The propagation of sound in three dimensional periodic lattices of solid-solid and solid-liquid composites is determined by calculating the acoustic band structure. Inclusion of viscous damping in the liquid is essential in understanding the acoustical properties of the solid-liquid composite when the characteristic length scale of the system matches the viscous penetration depth in the liquid. By using complex sound velocities to model the visco-elastic properties of the liquid we show that the solid-viscous liquid composite displays acoustic band gaps. These results are consistent with the hydrodynamic analysis of viscoelastic modes in porous media, binary mixtures and with a stratified layer model. As a characteristic example the ultrasonic band structure of silica spheres in ice and of colloidal crystals of silica in glycerol/water mixture are presented. Both systems display gaps under experimentally obtainable conditions.

Far infrared absorption spectra of some solid and fused alkalimetal nitrates

Abstract The low temperature spectra of solid films of sodium and cesium nitrate show very sharp absorption bands that must be ascribed to the transversal optical modes of the crystal lattice. With increasing temperature these bands broaden considerably but no special effect at the point of fusion is noticed.

Rotational Diffusion Model with a Variable Collision Distribution

Starting with an m‐diffusion model a matrix description is given of the rotational motion of a dipole molecule undergoing frequent collisions. This treatment gives rise to an analytical expression for the dipole correlation function and for the angular momentum correlation function in which a limited number of parameters from the model appear. It is argued that the collision distribution which determines the rotational diffusion process need not necessarily be a Poisson distribution. In liquids with strong interactions the distribution is governed by the frequency distribution of the medium. This leads to the inclusion of a librational motion in the rotational diffusion model. A comparison of simulations with different collision distributions and experimental data is given.

Colloidal aggregation in microgravity by critical Casimir forces

By using the critical Casimir force, we study the attractive strength dependent aggregation of colloids with and without gravity by means of near field scattering. Significant differences were seen between microgravity and ground experiments, both in the structure of the formed fractal aggregates as well as in the kinetics of growth. In microgravity purely diffusive aggregation is observed. By using the continuously variable particle interaction potential we can for the first time experimentally relate the strength of attraction between the particles and the structure of the aggregates.