Vibrofluidic Manipulation of Granular Fluids: Towards “Dry” Lab-on-a-Chip

Many “wet” techniques have been developed to manipulate liquids at small scales for lab-on-a-chip applications. An important and ubiquitous class of laboratory operations, however, involves dry techniques, where solid powders or other granular materials must be manipulated prior to addition of any liquids. To date, no facile techniques exist to manipulate dry granules in lab-on-a-chip devices. Here, we present a “vibrofluidic” technique to pump, mix, and separate granular fluids simply by vibrating a solid substrate. In contrast to existing vertical vibratory methods for fluidizing granular materials that engender no net motion, here we apply a horizontal, non-antiperiodic vibratory waveform, as is readily applied with a standard subwoofer. The direction and velocity of the flow are tuned by modulating the sign and amplitude, respectively, of the vibratory waveform, with typical pumping velocities on the order of centimeters per second. Different types of granules are mixed by combining them at Y-shaped junctions, and mixtures of granules with similar size but different friction coefficients can be separated by judicious choice of the vibratory waveform. We present asymptotic analyses of the granular fluid motion based on a frictional Froude number, and we discuss the implications for using vibrofluidics in conjunction with other established technologies for lab-on-a-chip and other applications.