Linear and Nonlinear Shock Attenuation of Aqueous Methylcellulose Solutions

Abstract

Aqueous methylcellulose solutions are fascinating inverse-freezing materials, known to reversibly form gels upon heating. Recently, these materials have been found to undergo this endothermic solidification upon impact. The impact-induced solidification was shown to occur in the microseconds’ timescale, setting the path for examining their functionality for shock absorption purposes. This present work focuses on characterizing the ability of methylcellulose solutions to mitigate impact forces, and on quantifying their attenuation coefficients for weak ultrasonic pulses as well as violent impacts. Ultrasonic attenuation measurements at temperatures higher than the gelation temperature (solid gel), reveal unique behavior which is attributed to the thermogelation mechanism. Impact experiments on 2 cm thick solutions have shown unexpectedly strong force and impulse mitigation, along with a high attenuation coefficient that grows exponentially with frequency. This attenuation performance was even further improved by increasing the concentration of gel-forming material. Reducing the thickness of the sample to 1 cm does not apparently reduce the force and impulse mitigation characteristics from those measured for 2 cm thickness, which is a distinct "anomaly" of materials used for wave mitigation. These observations imply that in the materials investigated herein, the main mechanism of shockwave attenuation is the sol→gel phase-transition, in contrast to shockwave passage through the viscous material bulk. Thus, the common normalization of the attenuation coefficient to the thickness of the medium is not valid for these materials.