Optical measurement of the shear stress and velocity distribution in an idealized deglutition process

Abstract

The rheological properties of food have an impact on physical interaction with various haptic sensors in the mouth during the chewing and swallowing process. Therefore, the investigations on food rheology are dedicated to improving the texture or mouthfeel of a product. It is advantageous to perform such rheological measurements in geometries and under conditions that reflect the flow conditions during the food distortion in the mouth cavity. Such investigations have been limited primarily to viscous or lumpy foodstuffs. Our study is dedicated to the compression of liquid foams in an idealized deglutition process. To reflect the flow conditions in such a process, we developed an experimental setup consisting of a quasi-two-dimensional replica of the palate and a movable tongue. During the idealized deglutition process, the optical measurements of bubble displacement and deformation were performed. The deglutition process was modelled by varying the compression speed of the tongue towards the static palate. The variation in mean bubble size and liquid fraction of foam for two tongue geometries with different roughness are available. It is observed that the velocity distribution of bubble motion during the deglutition corresponds to a rigid body flow, notwithstanding the surface roughness. The resulting wall-shear stress, however, was higher for the rough tongue plate for all foam bubble sizes and liquid fraction. The resulting velocity and wall-shear stress distributions can provide information about the haptic perception in the mouth during the swallowing process.