Peter W. Wilson

Heterogeneous nucleation of supercooled water, and the effect of an added catalyst

The statistics of liquid-to-crystal nucleation are measured rigorously by using a recently developed automated lag-time apparatus (ALTA). A single sample, in this case a sample of pure water both with and without an (insoluble) AgI crystal, is repeatedly cooled, nucleated, and thawed. Analysis of the data, coupled with a second kind of experiment, shows that the statistics of nucleation are consistent with a first-order kinetic mechanism over a wide range of supercooling temperatures. The limitations of classical nucleation theory are exhibited. Our analysis unifies many related experiments in biology, physics, chemistry, and chemical engineering.

Development of Sol–Gel Icephobic Coatings: Effect of Surface Roughness and Surface Energy

Sol-gel coatings with different roughness and surface energy were prepared on glass substrates. Methyl triethoxysilane (MTEOS), 3-Glycidyloxypropyl trimethoxysilane (GLYMO) and fluoroalkylsilane (FAS) were used to obtain a mechanically robust icephobic coating. Different amount of hydrophobic silica nano particles was added as fillers to introduce different roughness and surface energy to the coatings. The microstructure, roughness, and surface energy, together with elemental information and surface chemical state, were investigated at room temperature. The contact angle and sliding angle were measured at different temperatures to correlate the wetting behavior at low temperature with the anti-icing performance. The ice adhesion shear strength was measured inside an ice chamber using a self-designed tester. The factors influencing the ice adhesion were discussed, and the optimum anti-icing performance found in the series of coatings. It was found that lower surface energy leads to lower ice adhesion regardless of the roughness, while the roughness plays a more complicated role. The wetting behavior of the droplet on surface changes as temperature decreases. The anti-icing performance is closely related to the antiwetting property of the surfaces at subzero temperatures.

Stabilization of supercooled fluids by thermal hysteresis proteins.

It has been reported that thermal hysteresis proteins found in many cold-hardy, freeze-avoiding arthropods stabilize their supercooled body fluids. We give evidence that fish antifreeze proteins, which also produce thermal hysteresis, bind to and reduce the efficiency of heterogenous nucleation sites, rather than binding to embryonic ice nuclei. We discuss both possible mechanisms for stabilization of supercooled body fluids and also describe a new method for measuring and defining the supercooling point of small volumes of liquid.

Liquid-to-crystal nucleation: Automated lag-time apparatus to study supercooled liquids

The statistics of liquid-to-crystal nucleation are studied using an automated lag-time apparatus. A single 500 μL sample of distilled water is repeatedly supercooled to a fixed temperature below its equilibrium freezing temperature, held until freezing occurred, and then thawed. Our raw data is then a set of approximately 300 lag-times for each of three set supercooling temperatures. In each case, a small insoluble AgI crystal was added to ensure heterogeneous nucleation and average nucleation temperatures around ΔT=8 K. The distribution of lag-times is analyzed, and shown to be well approximated by a single exponential decay, with average lag-times in the range of 1000–3000 seconds. This average lag-time decreases markedly at deeper levels of supercooling, and for the present data, this decrease is fit equally well by exponential, power law decay, and classical nucleation functional forms.

Development of a high pressure automated lag time apparatus for experimental studies and statistical analyses of nucleation and growth of gas hydrates.

Nucleation in a supercooled or a supersaturated medium is a stochastic event, and hence statistical analyses are required for the understanding and prediction of such events. The development of reliable statistical methods for quantifying nucleation probability is highly desirable for applications where control of nucleation is required. The nucleation of gas hydrates in supercooled conditions is one such application. We describe the design and development of a high pressure automated lag time apparatus (HP-ALTA) for the statistical study of gas hydrate nucleation and growth at elevated gas pressures. The apparatus allows a small volume (≈150 μl) of water to be cooled at a controlled rate in a pressurized gas atmosphere, and the temperature of gas hydrate nucleation, T(f), to be detected. The instrument then raises the sample temperature under controlled conditions to facilitate dissociation of the gas hydrate before repeating the cooling-nucleation cycle again. This process of forming and dissociating gas hydrates can be automatically repeated for a statistically significant (>100) number of nucleation events. The HP-ALTA can be operated in two modes, one for the detection of hydrate in the bulk of the sample, under a stirring action, and the other for the detection of the formation of hydrate films across the water-gas interface of a quiescent sample. The technique can be applied to the study of several parameters, such as gas pressure, cooling rate and gas composition, on the gas hydrate nucleation probability distribution for supercooled water samples.

Type I Antifreeze Proteins Enhance Ice Nucleation above Certain Concentrations

In this study, we examined the effects that antifreeze proteins have on the supercooling and ice-nucleating abilities of aqueous solutions. Very little information on such nucleation currently exists. Using an automated lag time apparatus and a new analysis, we show several dilution series of Type I antifreeze proteins. Our results indicate that, above a concentration of ∼8 mg/ml, ice nucleation is enhanced rather than hindered. We discuss this unexpected result and present a new hypothesis outlining three components of polar fish blood that we believe affect its solution properties in certain situations.

Antifreeze glycopeptide adsorption on single crystal ice surfaces using ellipsometry

Antarctic fishes synthesise antifreeze proteins which can effectively inhibit the growth of ice crystals. The mechanism relies on adsorption of these proteins to the ice surface. Ellipsometry has been used to quantify glycopeptide antifreeze adsorption to the basal and prism faces of single ice crystals. The rate of accumulation was determined as a function of time and at concentrations between 0.0005 and 1.2 mg/ml. Estimates of packing density at saturation coverage have been made for the basal and prism faces.

Effect of solutes on the heterogeneous nucleation temperature of supercooled water: an experimental determination

We investigate the effect of solute concentration on the heterogeneous ice nucleation temperature (T(het)) of aqueous solutions of both NaCl and d-glucose. An automatic lag time apparatus (ALTA) technique allows the dependence of T(het) on solute concentration to be determined with statistical significance. Our results point to the solute-induced lowering of T(het) being a factor of two times the equivalent melting point depression at any fixed concentration, the same factor reported for homogeneous nucleation experiments with small molecular weight solutes.

Ice Premelting during Differential Scanning Calorimetry

Premelting at the surface of ice crystals is caused by factors such as temperature, radius of curvature, and solute composition. When polycrystalline ice samples are warmed from well below the equilibrium melting point, surface melting may begin at temperatures as low as -15 degrees C. However, it has been reported (Bronshteyn and Steponkus, 1993. Biophys. J. 65:1853-1865) that when polycrystalline ice was warmed in a differential scanning calorimetry (DSC) pan, melting began at about -50 degrees C, this extreme behavior being attributed to short-range forces. We show that there is no driving force for such premelting, and that for pure water samples in DSC pans curvature effects will cause premelting typically at just a few degrees below the equilibrium melting point. We also show that the rate of warming affects the slope of the DSC baseline and that this might be incorrectly interpreted as an endotherm. The work has consequences for DSC operators who use water as a standard in systems where subfreezing runs are important.

Microstructural-induced anisotropy in thin films for optical applications

Abstract Materials in thin-film form are much more interesting or much more trouble than their bulk counterparts, depending on your point of view. Most coatings are deposited at substrate temperatures considerably lower than the melting points of the evaporant materials, and the trend is generally toward even lower processing temperatures. In these films, the solid material is distributed in an array of fairly closely packed columns that run perpendicular to the substrate, and it is the columnar microstructure that has substantial influence over the physical behavior of the films. The microstructure is anisotropic and so are most thin film properties.