Abstract
Oil foams stabilized by crystallizing agents exhibit outstanding stability and show promise for applications in consumer products. The stability and mechanics imparted by the interfacial layer of crystals underpin product shelf life, as well as optimal processing conditions and performance in applications. Shelf life is affected by the stability against bubble dissolution over a long time scale, which leads to slow compression of the interfacial layer. In processing flow conditions, the imposed deformation is characterized by much shorter time scales. In practical situations, the crystal layer is therefore subjected to deformation on extremely different time scales. Despite its importance, our understanding of the behavior of such interfacial layers at different time scales remains limited. To address this gap, here we investigate the dynamics of single, crystal-coated bubbles isolated from an oleofoam, at two extreme time scales: the diffusion-limited time scale characteristic of bubble dissolution, ∼104 s, and a fast time scale characteristic of processing flow conditions, ∼10–3 s. In our experiments, slow deformation is obtained by bubble dissolution, and fast deformation in controlled conditions with real-time imaging is obtained using ultrasound-induced bubble oscillations. The experiments reveal that the fate of the interfacial layer is dramatically affected by the dynamics of deformation: after complete bubble dissolution, a continuous solid layer remains; after fast, oscillatory deformation of the layer, small crystals are expelled from the layer. This observation shows promise toward developing stimuli-responsive systems, with sensitivity to deformation rate, in addition to the already known thermoresponsiveness and photoresponsiveness of oleofoams.
Highlights
Oil foams have diverse use in pharmaceutical, food, and cosmetic applications.[1,2] While air−oil interfaces are difficult to stabilize with molecular surfactants,[3] various crystallizing agents have been found to confer long-term stability to oil foams[2−8] and novel air-in-oil-in-water systems.[9]
Much progress has been made in understanding the formation and stability of oil foams stabilized by crystallizing agents, yet our knowledge of the mechanical properties and dynamic behaviors of such systems remains limited, despite its importance for product shelf life, as well as optimal processing conditions and performance in applications
While there is significant literature available on the rheology of oleogels formed by the crystal network in the bulk oil phase,[1,2] and on the link between crystal formation and network properties,[13,14] less is understood about the microstructure and properties of the crystalstabilized interfaces.[15]
Summary
Oil foams have diverse use in pharmaceutical, food, and cosmetic applications.[1,2] While air−oil interfaces are difficult to stabilize with molecular surfactants,[3] various crystallizing agents have been found to confer long-term stability to oil foams[2−8] and novel air-in-oil-in-water systems.[9] Crystals can nucleate and grow both in the bulk oil phase and directly on the oil−air interface, upon decreasing the temperature of a preheated solution.[10] Crystals formed in the bulk can adsorb to the air−oil interfaces during mixing, while excess crystals can form a network in the oil phase (i.e., an oleogel).[5] In this case, foam stability is due to both the bulk and interfacial networks of crystals.[11,12] Much progress has been made in understanding the formation and stability of oil foams stabilized by crystallizing agents, yet our knowledge of the mechanical properties and dynamic behaviors of such systems remains limited, despite its importance for product shelf life, as well as optimal processing conditions and performance in applications. While there is significant literature available on the rheology of oleogels formed by the crystal network in the bulk oil phase,[1,2] and on the link between crystal formation and network properties,[13,14] less is understood about the microstructure and properties of the crystalstabilized interfaces.[15]
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