The Washburn capillary rise method is a standard technique for determining the wettability of powders expressed as a contact angle. The method requires the preparation of two identical beds of powder. One of these beds is used to follow the capillary rise with a perfectly wetting liquid (contact angle = 0) giving access to a bed structure parameter. The other bed is used with the liquid of interest (contact angle not = 0) and the capillary rise data is analysed using the previously determined structure parameter to obtain the contact angle. In the experiments reported here we have used a centrifugal packing technique to prepare beds of powder. This gives reproducible packings and also allows a certain degree of control of the bed porosity. In addition the air permeability of the beds is also determined prior to the capillary rise experiments. The results show that the value of the contact angle of a powder determined by the Washburn method depends on the porosity of the powder bed, and that the structure parameter can be determined from the air permeability using the Kozeny-Carman expression.

International audienceThis work examines the effect of the particle size of durum wheat semolina on the water diffusion coefficient and the evolution of the microstructure during hydration. Durum wheat semolina was sieved in 5 size categories. Water-particle interaction of powders (5 fractions and raw semolina) was studied using dynamic vapour sorption. Water sorption isotherms were fitted to G.A.B.. and Y&N models. Monolayer values as well as multilayer properties were calculated and compared. Absorption properties were determined by Y&N model. A sorption curve was modelled with the Fickian diffusion equation adapted to water diffusion into solid particles. A dynamic Fickian water diffusion coefficient was calculated for each particle size after conditioning at different RHs. The surface structure of conditioned fractions was observed by ESEM. The microstructure of semolina presented an evolution with the increase of size and RH. Starch grains presented an increased size and protein matrix had a different texture. After correlation of isotherm data and microstructure, the resulting state and properties of the components were a key to understand diffusion on food powder systems