Julien Dupas

Dynamic wetting on a thin film of soluble polymer: effects of nonlinearities in the sorption isotherm.

The wetting dynamics of a solvent on a soluble substrate interestingly results from the rates of the solvent transfers into the substrate. When a supported film of a hydrosoluble polymer with thickness e is wet by a spreading droplet of water with instantaneous velocity U, the contact angle is measured to be inversely proportionate to the product of thickness and velocity, eU, over two decades. As for many hydrosoluble polymers, the polymer we used (a polysaccharide) has a strongly nonlinear sorption isotherm φ(a(w)), where φ is the volume fraction of water in the polymer and aw is the activity of water. For the first time, this nonlinearity is accounted for in the dynamics of water uptake by the substrate. Indeed, by measuring the water content in the polymer around the droplet φ at distances as small as 5 μm, we find that the hydration profile exhibits (i) a strongly distorted shape that results directly from the nonlinearities of the sorption isotherm and (ii) a cutoff length ξ below which the water content in the substrate varies very slowly. The nonlinearities in the sorption isotherm and the hydration at small distances from the line were not accounted for by Tay et al., Soft Matter 2011, 7, 6953. Here, we develop a comprehensive description of the hydration of the substrate ahead of the contact line that encompasses the two water transfers at stake: (i) the evaporation-condensation process by which water transfers into the substrate through the atmosphere by the condensation of the vapor phase, which is fed by the evaporation from the droplet itself, and (ii) the diffusion of liquid water along the polymer film. We find that the eU rescaling of the contact angle arises from the evaporation-condensation process at small distances. We demonstrate why it is not modified by the second process.

Powder wettability at a static air–water interface

The reconstitution of a beverage from a dehydrated powder involves several physical mechanisms that determine the practical difficulty to obtain a homogeneous drink in a convenient way and within an acceptable time for the preparation of a beverage. When pouring powder onto static water, the first hurdle to overcome is the air-water interface. We propose a model to predict the percentage of powder crossing the interface in 45 s, namely the duration relevant for this application. We highlight theoretically the determinant role of the contact angle and of the particle size distribution. We validate experimentally the model for single spheres and use it to predict the wettability performance of commercial food powders for different contact angles and particles sizes. A good agreement is obtained when comparing the predictions and the wettability of the tested powders.

Reconstitution Properties of Sucrose and Maltodextrins

Carbohydrates such as sucrose and maltodextrins are commonly used in dehydrated food beverages. However, these ingredients may have, in some cases, negative impacts on the reconstitution performance (e.g., lump formation), compromising key consumers expectations. In this study, we propose to discuss the performance of carbohydrates with regard to major physical steps of reconstitution (wetting, capillarity, dispersion, and dissolution). We show how particle size and water temperature drive the kinetics of dissolution of crystalline sucrose and propose descriptive equations. For amorphous maltodextrin, we quantify variations in wetting, capillarity, and dissolution performance as a function of important solid properties (moisture content, molecular weight, and particle size) as well as the liquid temperature. By doing so, we highlight the important role of the glass-transition temperature in relation to the moisture content of the powder. The comprehensive understanding provided by this work may be used to optimize product formulation in term of reconstitution performance.