Colloidal gels made of carbon black particles dispersed in light mineral oil are “rheo-acoustic” materials, i.e., their mechanical and structural properties can be tuned using high-power ultrasound, sound waves with submicrometer amplitude and frequencies larger than 20 kHz. The effects of high-power ultrasound on the carbon black gel are demonstrated using two experiments: rheology coupled to ultrasound to test for the gel mechanical response and a time-resolved ultra-small-angle x-ray scattering experiment (TRUSAXS) coupled to ultrasound to test for structural changes within the gel. We show that high-power ultrasound above a critical amplitude leads to a complex viscoelastic transient response of the gels within a few seconds: a softening of its storage modulus accompanied by a strong overshoot in its loss modulus. Under high-power ultrasound, the gel displays a viscoelastic spectrum with glasslike features and a significant decrease in its yield strain. Those effects are attributed to the formation of intermittent microcracks in the bulk of the gel as evidenced by TRUSAXS. Provided that the shear rate is not large enough to fully fluidize the sample, high-power ultrasound also facilitates the flow of the gel, reducing its yield stress as well as increasing the shear-thinning index, thanks again to the formation of microcracks.

中文翻译：

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高功率超声作用下炭黑凝胶的力学与结构

由分散在轻质矿物油中的炭黑颗粒制成的胶体凝胶是“流变声”材料，即可以使用高功率超声波，亚微米振幅和大于20 kHz的频率的声波来调整其机械和结构特性。使用以下两个实验证明了高功率超声对炭黑凝胶的影响：流变学与超声耦合以测试凝胶的机械响应以及时间分辨的超小角度X射线散射实验（TRUSAXS）与超声耦合测试凝胶中的结构变化。我们表明，超过临界振幅的高功率超声会导致凝胶在几秒钟内发生复杂的粘弹性瞬态响应：其储能模量软化并伴随其损耗模量的强烈过冲。在大功率超声波下 凝胶显示出具有玻璃状特征的粘弹性光谱，并且其屈服应变显着降低。这些影响归因于TRUSAXS证明，在凝胶的大部分中形成了间歇性微裂纹。假设剪切速率不足以使样品完全流化，高功率超声还可以促进凝胶的流动，降低凝胶的屈服应力，并增加剪切稀化指数，这也要归功于微裂纹的形成。