Recently, the phenomena of streaming suppression and relocation of inhomogeneous miscible fluids under acoustic fields were explained using the hypothesis on mean Eulerian pressure. In this work, we derive the expression for the acoustic body force without relying on any prior assumptions regarding the second-order Eulerian pressure. We present a theory of nonlinear acoustics for inhomogeneous fluids from first principles, which explains streaming suppression and acoustic relocation in both miscible and immiscible inhomogeneous fluids inside a microchannel. This theory predicts the relocation of higher impedance fluids to pressure nodes of the standing wave, which agrees with recent experiments.

We present a technique for mixing the fluids in a microchannel using ultrasonic waves. Acoustic mixing is driven by the acoustic body force, which depends on the density gradient and speed of the sound gradient of the inhomogeneous fluid domain. In this work, mixing of fluids in a microchannel is achieved via an alternating multinode mixing method, which employs acoustic multinode standing waves of time-varying wavelengths at regular time intervals. The proposed technique is rapid, efficient, and found to enhance the mixing of fluids significantly. It is shown that the mixing time due to acoustic mixing (2–3 s) is reduced by two orders of magnitude compared to the mixing time only due to diffusion (400 s). Furthermore, we investigate the effects of the acoustic mixing on different fluid flow configurations and sound wave propagation directions as they have a direct influence on mixing time and have rarely been addressed previously. Remarkably, it is found that mixing performance is strongly dependent on the direction of the acoustic wave propagation. The acoustic field propagated parallel to the fluid-fluid interface mixes fluids rapidly (2–3 s) as compared to the acoustic field propagated perpendicular to the fluid-fluid interface (40 s).

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This work investigates the effects of actuation frequency and fluid properties on the relocation of miscible inhomogeneous fluids inside a microchannel under acoustic standing waves. Remarkably, we demonstrate the cases in which relocation is achieved relatively faster in the case of fluids with smaller impedance difference between them (1.5%), than in the case of fluids with larger impedance difference (10%) when actuated at an optimum frequency. Subsequently, we show that, if the impedance difference between two fluids is less, actuation of the microchannel at a single frequency is sufficient for the fast and complete relocation, whereas, in the case of larger impedance difference between the fluids, sweeping at multiple frequencies results in fast and more complete relocation compared to actuation of the system by a single frequency. Furthermore, the role of gravity in the process is also analyzed in detail.