A mesoscale study of creep in a microgel using the acoustic radiation force

Abstract

We study the motion of a sphere of diameter 330 µm embedded in a Carbopol microgel under the effect of the acoustic radiation pressure exerted by a focused ultrasonic field. The sphere motion within the microgel is tracked using videomicroscopy and compared to conventional creep and recovery measurements performed with a rheometer. We find that under moderate ultrasonic intensities, the sphere creeps as a power law of time with an exponent α 0.2 that is significantly smaller than the one inferred from global creep measurements below the yield stress of the microgel (α 0.4). Moreover, the sphere relaxation motion after creep and the global recovery are respectively consistent with these two different exponents. By allowing a rheological characterization at the scale of the sphere with forces of the order of micronewtons, the present experiments pave the way for acoustic "mesorheology" which probes volumes and forces intermediate between standard macroscopic rheology and classical microrheology. They also open new questions about the effects of the geometry of the deformation field and of the sphere size and surface properties on the creep behaviour of soft materials.

Acoustic Bubble Dynamics in a Yield-Stress Fluids

“…Creep is however usually associated to irreversible strain and a non-linear stress-strain relation in bulk rheology experiments, both of which are not observed here. Interestingly, fully reversible creep motion up to the yield point has been reported in experiments in which acoustic radiation forces are used to push small spheres [50]. The relatively small pressure gradients applied in our experiments according to Equation ( 11) then cannot alone initiate bubble rise.…”

“…We have used in Section IV C the constant (or zero-frequency) part of the acoustic radiation forces -the Bjerknes forcesto perform an equivalent of step-stress tests, but at a local scale R 0 and on a relatively short timescale N/ f 0.05 s. For moderate acoustic stresses σ ac ≤ σ Y , we measure a linear strain-stress relation at the end of oscillations, from which we deduce an independent measurement of the local linear elastic modulus of the fluid below yielding, G = 44.4 ± 3.5 Pa, comparable to that obtained using bulk rheology, G = 36 Pa. All quantities used to derive G are either directly measured or estimated from the resonance curve: hence, in contrast with previous works [50], our measurement is truly independent from bulk rheology. The complex time dependence of the displacement shown in Figure 4(a) is reminiscent of creep behaviour [5].…”

Acoustic bubble dynamics in a yield-stress fluid

“…Interestingly, fully reversible creep motion up to the yield point has also been reported in experiments in which acoustic radiation forces are used to push small spheres. 53 The relatively small pressure gradients applied in our experiments according to eqn (11) then cannot alone initiate bubble rise. Performing experiments of longer duration may reveal whether the response to acoustic radiation forces indeed follows a power law or an exponential profile with time, which could be helpful to validate the recent, advanced models of yield-stress fluids.…”

“…We have used in Section 4.3 the constant (or zero-frequency) part of the acoustic radiation force to perform an equivalent of step-stress tests, but at a local scale R 0 . For moderate acoustic stresses s ac r s Y , we measure a linear strain-stress relation at the end of oscillations, from which we deduce an independent measurement of the local linear elastic modulus of the fluid below yielding, G = 44.4 AE 3.5 Pa, comparable to that obtained using bulk rheology, G = 36 Pa. All quantities used to derive G are either directly measured or estimated from the resonance curve: hence, in contrast with previous works, 53 our measurement is truly independent from bulk rheology.…”

The effect of microparticles/nanoparticles surface modification on the magnetorheological fluid properties: A review

“…Smart fluids, on the other hand, are classified into different categories according to their stimuli and responses. So far, various types of smart materials such as electrorheological fluids (ERF) (Dong et al, 2019;Liu and Choi, 2012b;Wen et al, 2008), magnetorheological fluids (MRF) (Phule, 1999;Rodrı´guez-Arco et al, 2013;Ronzova et al, 2021), ferrofluids (FFs) (Li et al, 2018(Li et al, , 2019(Li et al, , 2020Rodrı´guez-Arco et al, 2014;Usadel et al, 2019), magnetorheological gel (MRG) (An et al, 2010;Lidon et al, 2019;Pang et al, 2020), magnetorheological foam (Makarova et al, 2019;Muhazeli et al, 2021), and magnetorheological elastomer (MRE) (Bastola et al, 2020;Snarskii et al, 2021;Wen et al, 2020;Yu et al, 2015) have been identified. In some cases, the use of these materials has led to commercial-scale production by BASF (Munoz et al, 1999), Lord crop (Andrew et al, 2008), Delfi (Iyengar and Foister, 2003, 2004a, 2004b, and Ford (Ginder et al, 1996) company.…”