Frictional separation of granular materials in opposing directions via horizontal vibrations

Current methods of separating mixtures of granular materials rely on differences in the size, shape, or density of the respective granules. Here we report a method to separate mixtures of granular materials based on differences in the frictional coefficients of the granules, simply by placing the mixture on a horizontally vibrating flat plate. Our numerical calculations reveal a regime of dual-mode vibratory waveforms where granules with different granule-substrate friction coefficients simultaneously move in opposite directions in response to the same waveform. Our experimental observations confirm this prediction: random piles of (i) metal washers and rubber slices, (ii) salt and pepper, (iii) pistachio kernels and shells, and (iv) healthy and defected coffee beans, are all observed to separate with 100% efficiency in response to appropriate vibratory waveforms. We further demonstrate that the observed separation velocities are consistent with numerical calculations predicated solely on frictional interactions with the substrate, and we discuss the implications for friction-based separations of granular materials both at large industrial scales as well as in lab-on-a-chip devices.