Spreading of a granular droplet under horizontal vibrations

Vibrational manipulation of dry granular materials in lab-on-a-chip devices

We present vibrational techniques to pump, mix, and separate dry granular materials using multifrequency vibrations applied to a solid substrate with a standard audio system. The direction and velocity of the granular flow are tuned by modulating the sign and amplitude, respectively, of the vibratory waveform, with typical pumping velocities of centimeters per second. Different granular materials are mixed by combining them at Y-shaped junctions, and mixtures of granules with different friction coefficients are separated along straight channels by judicious choice of the vibratory waveform. We demonstrate that the observed velocities accord with a theory valid for sufficiently large or fast vibrations, and we discuss the implications for using vibrational manipulation in conjunction with established microfluidic technologies to combine liquid and dry solid handling operations at sub-millimeter length scales.

Laboratory experiment and discrete-element-method simulation of granular-heap flows under vertical vibration

Granular flow dynamics on a vertically vibrated pile is studied by means of both laboratory experiments and numerical simulations. As already revealed, the depth-averaged velocity of a fully fluidized granular pile under strong vibration, which is measured by a high-speed laser profiler in the experiment, can be explained by the nonlinear diffusion transport model proposed by our previous paper [Tsuji et al., Phys. Rev. Lett. 120, 128001 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.128001]. In this paper, we report that a similar transport model can be applied to the relation between the surface velocity and slope in the experiment. These facts are also reproduced by particle-scale numerical simulations based on the discrete element method. In addition, using these numerical results, the velocity profile inside the fluidized pile is measured. As a result, we show that the flow velocity decreases exponentially with depth from the surface of the pile, which means that a clearly fluidized region, also known as shear band structure, is localized around the surface. However, its thickness grows proportionally with the local height of the pile, i.e., the shear band does not consist of a fluidized layer with a constant thickness. From these features, we finally demonstrate that the integration of this exponentially decreasing velocity profile is consistent with the depth-averaged velocity predicted by the nonlinear diffusion transport model.