Abstract
Colloidal gels, where nanoscale particles aggregate into an elastic yet fragile network, are at the heart of materials that combine specific optical, electrical, and mechanical properties. Tailoring the viscoelastic features of colloidal gels in real time thanks to an external stimulus currently appears as a major challenge in the design of “smart” soft materials. Here we introduce “rheoacoustic” gels, a class of materials that are sensitive to ultrasonic vibrations. By using a combination of rheological and structural characterization, we evidence and quantify a strong softening in three widely different colloidal gels submitted to ultrasonic vibrations (with submicron amplitude and frequency 20–500 kHz). This softening is attributed to micron-sized cracks within the gel network that may or may not fully heal once vibrations are turned off depending on the acoustic intensity. Ultrasonic vibrations are further shown to dramatically decrease the gel yield stress and accelerate shear-induced fluidization. Ultrasound-assisted fluidization dynamics appear to be governed by an effective temperature that depends on the acoustic intensity. Our work opens the way to a full control of elastic and flow properties by ultrasonic vibrations as well as to future theoretical and numerical modeling of such rheoacoustic gels.
7 More- Received 20 June 2019
- Revised 14 October 2019
- Accepted 19 December 2019
DOI:https://doi.org/10.1103/PhysRevX.10.011028
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Colloidal gels are a class of soft materials with a number of applications, including paints, oil extraction, pharmaceuticals, and food products. These gels are typically formed from many attractive nanoscale particles whose collective strength provides the system with elastic properties. Controlling these elastic properties in real time is a key pathway to the design of smart, responsive materials. Here, we show how to use ultrasonic vibrations to control the elasticity and yield stress of a colloidal gel.
We subject three wildly different colloidal gels to ultrasonic vibrations, ranging in frequency from 20 to 500 kHz, and find that all three gels soften considerably. This softening—a decrease in the elasticity of the materials—is attributed to the formation of microcracks within the gel network when the ultrasound is turned on. We also show that ultrasound can dramatically decrease the gel yield stress (the minimum stress necessary to break the gel) and accelerate fluidization under stress.
Our results open the door to full control of the elastic and flow properties of colloidal gels by ultrasonic vibrations.