Vibration-induced geometric patterns of persistent holes in Carbopol gels

Abstract

New results are presented for the behavior of viscoplastic, aqueous polymer gels of Carbopol 940 when subjected to vertical, mechanical vibration. The vibrations were at frequencies in the range 40–100Hz, and at accelerations of 5–35 times that of gravity. It is shown that, upon vibration, samples form persistent holes in a regular, geometric pattern. Because the gels are viscoplastic, the holes persist even when vibration ceases. Depending on the sample mass and vibration conditions, 1–7 holes form at regular angular intervals, and at a constant radial distance from the sample center. Increasing the sample mass can lead to the formation of a second ring of holes, at a larger radius, and at angular positions midway between those in the inner ring. The holes form reproducibly over a range of accelerations, with the critical acceleration for the onset of hole formation increasing linearly with the vibration frequency. Qualitatively the hole arrangement is surprisingly similar to two-dimensional surface shapes derivable from lubrication theory of a Newtonian fluid that has been modified to include a destabilizing rheological hysteresis in response to the vibration.