Wetting of Soluble and Insoluble Heterogeneous Food Surfaces
Creators: name: MELIDES, SOLOMON
Publisher: name: University of Surrey
Year: 2022

description: "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, all of which impede reconstitution of food products such as soups, beverages and instant baby formula.
description: To investigate the effects surface fat plays on advancing contact lines, and thus the rehydration faults outlined above, this research begins by investigating a simplified system of an advancing contact line encountering periodic defects. This is to understand the cumulative effects of closely ordered defects on a moving contact line. This understanding is then used to evaluate a novel model system of a food powder surface so as to better reflect what is observed in real food products.
description: The first part of the study focused on the quasi-static characterisation of the pinning force and of the deformation of the interface induced by periodic defects (holes) on insoluble surfaces made from steel. As yet there are no systematic studies into the synergistic effects of periodic defects on advancing contact lines. This research is the first to investigate the cumulative defect effects. The pinning force was found to be linear with depth until the pinning energy is overcome by the contact line (CL) elasticity. Four-phase CLs reduce the pinning force. Offsetting the defects caused a decrease in pinning force. As the periodicity (d ) decreased, the cumulative de-pinning force increased, though non linearly. That is to say that the cumulative effects of neighbouring defects decreased the individual contribution to the total pinning force. This is more evident in defects of larger radii wetted by liquids with lower interfacial tensions: for r = 0.56 mm, halving d/r led to a 30 % decrease in the depinning force per hole for both 30 % and 50 % EtOH solutions. The depinning depth was found to decrease to a value approximated to the defect diameter, as d/r exceeds unity. Contact line depression was not found to be impacted by the defect radius or by the wetting liquid. However, decreasing the change in contact angle by using cocoa butter reduced the contact line depression and in most instances, the contact line recovered to the meniscus height of the substrate in the periodicities studied.
description: A theoretical model, developed at the University of Oxford by Professor Dominic Vella, was compared favourably with the experimental results. The theory describes the pinning force induced by a periodic array of holes on the interface and the resulting deformation of the interface. The theory also predicts the interfacial stiffness and allows one to predict the pinning force for voids for small interfacial deflections. As the interfacial deflection increases, the model begins to over-predict the pinning force though remains comparable to the experimental data, even at depths close to the depinning position. The contact line shape from the model matches well the experimental values at small interfacial deflections. However, greater differences between the model and experimental values are observed in the contact line depression.
description: In the second part of the study, the wetting of soluble maltodextrin thin films patterned with hydrophobic cocoa butter defects (deposited by inkjet printing) of various area fractions (AF ) and sizes, was studied. Using a novel model system, this research investigated the impact of hydrophobic (cocoa butter) defects on the advancing contact lines (CL) of water on water-soluble (maltodextrin). Previous work on the dissolution of food powders have been mainly qualitative, investigating particle-particle interactions. As yet there have been no systematic studies on the wetting of heterogeneous powder surfaces. The new model powder surface incorporates real food product materials which can be used as a realistic proxy for the wetting of food powder surfaces. Hydrophobic deposits with AF &gt;2 % were seen to impede the spreading of water. Increasing AF was found both to increa"