Nonlinear Friction Dynamics of Oil-in-Water and Water-in-Oil Emulsions on Hydrogel Surfaces

Abstract

In this study, the friction properties of emulsions in an oral environment were investigated to understand the food-texture recognition mechanisms occurring on biological surfaces. Numerous publications have suggested that the friction phenomena depend on friction conditions, such as the surface characteristics, as well as the shape and movement of contact probes. Traditional friction evaluation systems are unsuitable for mimicking the oral environment. Thus, in this study, the friction forces between two fractal agar gel substrates in an emulsion were examined using a sinusoidal motion friction evaluation system that effectively mimics the oral environment. The physical properties of the fractal agar gel, including the elasticity, hydrophilicity, and surface roughness, were analogous to those of the human tongue. Furthermore, the sinusoidal motion imitated the movements of living organisms. Depending on the samples, three friction profiles were observed. For water, the surfactant aqueous solution, and olive oil, the friction profiles of the outward and homeward processes were symmetric (stable pattern). Interestingly, for an oil-in-water (O/W) emulsion, friction behaviors with not only an asymmetric friction profile (unstable pattern I) but also a lubrication phenomenon, which temporarily decreased the friction force (unstable pattern II), were noted. The probability for the appearance of unstable patterns and adhesion force between the gel substrates increased with the oil content of the O/W emulsions. These characteristic friction phenomena were attributed to the strong adhesive force in the emulsion, which was sandwiched between the agar gel substrates. The findings obtained in this study would contribute significantly to understanding the food-texture recognition mechanisms and dynamic phenomena occurring on biological surfaces.

Friction Dynamics of Foams under Nonlinear Motion

“…We observed that the δ was affected by various factors such as the hardness and thickness of the material and viscosity of the fluid. 16,18,19 Conversely, in the case of surfactant aqueous solution 2, static friction and a sharp increase in friction force were observed (Figure 2b). In the outward direction, no static friction was observed, and the friction force increased to 0.25 N during the kinetic friction process.…”

“…The various dissipative modes of foam cause a decrease in the effective friction coefficient as sliding speed increases. It is possible that the viscous dissipation ,, and the dissipation related to surfactant diffusion affect the friction dynamics of foam − because a thick lubricating film was formed between the plates. In addition, the dissipation related to wall slip of bubbles is one of the most important modes because a smooth acrylic plate was used in this study.…”

Friction Dynamics of Organogels on Artificial Skin Surfaces

“…Key words: organogel, wax, friction, hardness, gelation Y. Nonomura, H. Ogura, H. Niino et al served 11) . In the friction of bubbles between acrylic plates, static friction does not occur, and a friction profile, in which the friction force increases according to velocity, is observed 12) .…”

Characterization of food emulsions and dispersions based on nonlinear friction dynamics

“…Therefore, we developed a sinusoidal‐motion friction evaluation system that combines a sinusoidal‐motion friction evaluation device and fractal agar gel to mimic the physical characteristics of the wet, soft, hydrophilic, and rough tongue, buccal meatus and palate and the motion of the tongue with acceleration (Aita et al, 2017; Kikuchi et al, 2021; Kikuchi et al, 2022; Okawara et al, 2020). The elastic modulus of the fractal agar gel was 0.90 ± 0.06 × 10 6 Pa.…”