Scattering from small colloidal particles in a semidilute polymer solution

The correlations between the segments of a semidilute polymer solution are found to induce correlations in the positions of small particles added to the solution. Small means a diameter much less than the polymer's correlation length. In the presence of polymer the particles behave as if they attracted each other. It is shown how the polymer's correlation length may be determined from a scattering experiment performed on the spheres.Comment: 12 pages, 3 figure

Generalisation of Levine's prediction for the distribution of freezing temperatures of droplets: A general singular model for ice nucleation

Models without an explicit time dependence, called singular models, are widely used for fitting the distribution of temperatures at which water droplets freeze. In 1950 Levine developed the original singular model. His key assumption was that each droplet contained many nucleation sites, and that freezing occurred due to the nucleation site with the highest freezing temperature. The fact that freezing occurs due to the maximum value out of large number of nucleation temperatures, means that we can apply the results of what is called extreme-value statistics. This is the statistics of the extreme, i.e., maximum or minimum, value of a large number of random variables. Here we use the results of extreme-value statistics to show that we can generalise Levine's model to produce the most general singular model possible. We show that when a singular model is a good approximation, the distribution of freezing temperatures should always be given by what is called the generalised extreme-value distribution. In addition, we also show that the distribution of freezing temperatures for droplets of onesize, can be used to make predictions for the scaling of the median nucleation temperature with droplet size, and vice versa.Comment: 9 pages, 2 figure

The coil-globule transition of polymers of long rigid monomers connected by flexible spacers

A simple model of a polymer with long rigid segments which interact via excluded volume repulsions and short ranged attractions is proposed. The coil-globule transition of this model polymer is strongly first order, the globule is crystalline and the coil which coexists with the globule is swollen. A virial expansion truncated at low order is shown to provide a very poor approximation to the free energy and so a cell theory is used to calculate the free energy of the globule.Comment: 9 pages, 2 figure

Metastability and nucleation in the dilute fluid phase of a simple model of globular proteins

The dilute fluid phase of model globular proteins is studied. The model possesses a fluid-fluid transition buried within the fluid-crystal coexistence region, as do some globular proteins. If this fluid-fluid transition is not buried deep inside the fluid-crystal coexistence region the crystalline phase does not nucleate within the dilute fluid. We link this lack of nucleation of the crystal to the interactions in our model and speculate that similar interactions between globular proteins are responsible for the difficulty found in crystallising many globular proteins.Comment: 11 pages, 4 figure

Classical nucleation theory for the nucleation of the solid phase of spherical particles with a short-ranged attraction

Classical nucleation theory is used to estimate the free-energy barrier to nucleation of the solid phase of particles interacting via a potential which has a short-ranged attraction. Due to the high interfacial tension between the fluid and solid phases, this barrier is very large, much larger than in hard spheres. It is divergent in the limit that the range of the attraction tends to zero. We predict an upper limit on nucleation in good agreement with the results of experiments on the crystallisation of proteins.Comment: 10 pages including 5 figure

Nucleation of a new phase on a surface that is changing irreversibly with time

Nucleation of a new phase almost always starts at a surface. This surface is almost always assumed not to change with time. However, surfaces can roughen, partially dissolve and change chemically with time. Each of these irreversible changes will change the nucleation rate at the surface, resulting in a time-dependent nucleation rate. Here we use a simple model to show that partial surface dissolution can qualitatively change the nucleation process, in a way that is testable in experiment. The changing surface means that the nucleation rate is increasing with time. There is an initial period during which no nucleation occurs, followed by relatively rapid nucleation.Comment: 5 pages, 3 figures. Supplementary Movie 1 at http://personal.ph.surrey.ac.uk/~phs1rs/papers/supp_movie1_pre.mp4, description at http://personal.ph.surrey.ac.uk/~phs1rs/papers/supp_mat_pre14.pd

Depletion driven adsorption of colloidal rods onto a hard wall

In a mixed suspension of rods and small polymer coils, the rods adsorb onto a hard wall in contact with the suspension. This adsorption is studied in the low density of rods limit. It is driven by depletion forces and is much stronger for long rods than for spheres. This is shown by means of exact, numerical, calculations and an approximate theory.Comment: 11 pages, 5 figure

Stratification of mixtures in evaporating liquid films occurs only for a range of volume fractions of the smaller component

I model the drying of a liquid film containing small and big colloid particles. Fortini et al. [A. Fortini et al, Phys. Rev. Lett. 116, 118301 (2016)] studied these films with both computer simulation and experiment. They found that at the end of drying the mixture had stratified with a layer of the smaller particles on top of the big particles. I develop a simple model for this process. The model has two ingredients: arrest of the diffusion of the particles at high density, and diffusiophoretic motion of the big particles due to gradients in the concentration of the small particles. The model predicts that stratification only occurs over a range of initial concentrations of the smaller colloidal species. At concentrations that are either too low or too high, the concentration gradients due to drying are not enough to push the big particles away and so produce a layer at the top of only small particles. In agreement with earlier work, the model also predicts that large Peclet numbers for drying are needed to see stratification.Comment: 8 pages, 5 figure

Phase behaviour of a simple model of globular proteins

A simple model of globular proteins which incorporates anisotropic attractions is proposed. It is closely related to models used to model simple hydrogen-bonding molecules such as water. Theories for both the fluid and solid phases are presented, and phase diagrams calculated. The model protein exhibits a fluid-fluid transition which is metastable with respect to the fluid-solid transition for most values of the model parameters. This is behaviour often observed for globular proteins. The model offers an explanation of the difficulty observed in crystallising some globular proteins and suggests that some proteins may not have a solid phase at all under all but extreme conditions.Comment: 12 pages including 5 figures Error in B2 of vdW fluid correcte