Understanding the friction phenomena on a gel surface under accelerated conditions is important for the designing of functional materials. However, there are few reports on friction under such conditions. In the present study, the effects of velocity, normal force, and gel hardness on the friction force were evaluated between two agar gels under sinusoidal motion. We found a friction phenomenon with an extremely low friction coefficient on the gel surfaces: the friction coefficient became less than 0.02 when sliding velocity increased. In addition, the profile of the friction coefficient was different between outward and homeward processes in the reciprocating sliding motion. In the outward direction, the low friction coefficient was maintained even if the sliding velocity decreased. On the other hand, the friction coefficient increased with sliding velocity in the homeward direction. This characteristic friction profile is caused by a long relaxation time on the gel surfaces. When the gel substrate is rubbed for a shorter time than the relaxation time, the morphology of the gel surface becomes unstable. Under such conditions, the formation and extinction of a thick liquid film can induce a super lubrication state and the asymmetric friction phenomena. These findings are useful not only for developing functional materials but also for understanding nonequilibrium phenomena in soft biological systems.

No abstract

No abstract

Interfacial phenomena on soft and wet materials, such as hydrogels, are important for modeling physical phenomena, such as friction, wetting, and adhesion on hydrophilic biosurfaces. Interfacial phenomena on soft material surfaces are not only affected by the properties of the surface but also by the geometry of the substrate. However, there are few reports on the influence of geometry and deformability on friction behavior at gel interfaces. In this study, we evaluate the effects of the thickness ( H ) of the upper agar gel layer on the friction force between gels under a sinusoidal movement. Although H does not significantly affect the friction force or pattern, the normalized delay time (δ), which is the normalized time lag in the friction force response to the contact probe’s movement, increases with H . A regression analysis between δ and H shows that δ increased linearly with H . We present a simple model incorporating a shear modulus to qualitatively explain the experimental results. The analysis and our model indicate that one must not only consider surface properties, such as adhesion, but also thickness and rigidity when studying friction behavior at the gel–surface interface. These findings will be useful for understanding friction phenomena on soft biological systems, such as the tongue, throat, esophagus, and gut surfaces.

"Thickness" is one of the descriptors of texture in liquid and semisolid foods. In this study, friction in thickener aqueous solutions was evaluated, using a biomimetic friction evaluation system, to show the correlation between friction data and sensory thickness and the recognition mechanism of this sensation during the process of eating. This system can measure friction forces under sinusoidal movement on fractal agar gel, which mimics the morphology and physical properties of the tongue. We found that an increase in the viscosity of the thickener aqueous solution was responsible for both the sensory score of thickness and the asymmetric profile of the friction coefficient in a reciprocating motion. In the case of low viscosity liquids such as water, many of the subjects did not feel thickness, and the friction profile "stable pattern I," that is, a static friction coefficient larger than kinetic friction and a similar profile in the outward and inward processes were observed. However, in the case of solutions containing 3 or 5 wt% of food thickener, the friction profile "unstable pattern I," that is, different friction behaviors in the outward and inward processes, was observed, and many of the subjects experienced strong thickness. In addition, the static friction coefficient at the first cycle was small, and the changes of friction coefficient by the reciprocating motion being repeated was large. These friction phenomena can occur in the mouth and are expected to induce sensory thickness.