P2: Impacts of surface defects on the dynamics of wetting and on the shape of advancing contact lines

Solomon Melides, Joseph Keddie, Dominic Vella, and Marco Ramaioli

University of Surrey, UK

Food powder dissolution is, in many cases, a blight for consumers and manufacturers alike. Surface fat is known to cause agglomeration, clumping, lumping and islands, which all impede reconstitution.

This research investigated the impacts of hydrophobic (cocoa butter) defects on the advancing contact lines (CL) of water on water-soluble (maltodextrin) and insoluble (steel/glass) surfaces. We provide a theoretical model describing the interactions a periodic array of voids have on the force and interfacial deflection of advancing CL as a defect is overcome. The pinning force is linear with depth until the pinning energy is overcome by the CL elasticity. Four-phase CL reduce the pinning force.

The wetting of soluble maltodextrin films patterned with hydrophobic defects (by inkjet materials printing) of various area fractions (AF) and sizes was assessed over 80s. Hydrophobic droplets with AF even as low as 1% were seen to impede the wetting of water. Increasing AF was found both to increase the onset and end contact angles (θ) and to reduce the contact line speeds (U). AFs of ~25% were found to triple the onset θ and to reduce U by half. As AF increased, stepwise undulations in θ were observed, indicating that U reduced locally as the contact line encountered defects. Increasing the defect droplet size (but maintaining AF) was seen not to greatly impact θ and U. Randomising the placement of the droplets was also not seen to significantly impact the wetting of the films. This research begins to explain how defects and their interactions impede wetting of food powders, identifying the key parameters that control wetting and depinning, which can help improving food powder wetting performance.

Key dates

Registration deadline:

31 January 2022

Organised by the IOP Food Physics Group