Control of acoustic streaming can significantly impact fluid and particle transport in microfluidics. We report enhancement, suppression, and reversal of acoustic streaming inside a rectangular microchannel by controlling the fluid viscoelastic properties. Our study reveals that the streaming regimes depend on Deborah number ($De$) and viscous diffusion number ($Dv$), expressed in terms of a Streaming Coefficient ($C_s$). We find streaming is enhanced when $C_s>1$, suppressed for $0\leq C_s\leq1$, and reversed when $C_s<0$. We explain the regimes in terms of the interplay between the Reynolds and viscoelastic stresses that collectively drive fluid motion. Remarkably, we discover the role of viscoelastic shear waves in acoustic streaming transition characterized by the ratio of acoustic attenuation length and shear wavelength. We gain deeper insight into the streaming transition by examining energy dynamics in terms of the loss and storage moduli. Our study may find applications in acousto-microfluidics systems for particle handling and fluid pumping/mixing.
Oscillatory motion in a fluid, driven by acoustic waves or vibrating boundaries, produces a steady flow known as acoustic streaming (Faraday 1831;Lighthill 1978;Eckart 1948;Schlichting 1932;Rayleigh 1884). This nonlinear effect stems from the dissipation of acoustic energy (Lighthill 1978) and arises through two primary mechanisms. In large-scale systems, where dimensions far exceed the acoustic wavelength, bulk attenuation of traveling waves leads to "quartz wind" (Eckart 1948). In contrast, in small-scale systems, energy dissipation within a thin boundary layer gives rise to "boundary layer-driven streaming" (Schlichting 1932;Rayleigh 1884). The vorticity generated within the boundary layer, known as "inner streaming" or "Schlichting streaming" (Schlichting 1932), drives counter-rotating vortices in the bulk fluid, referred to as "outer streaming" or "Rayleigh streaming" (Rayleigh 1884). Boundary-layer-driven streaming has gained attention in acousto-microfluidic systems for applications in pumping, mixing, and particle/cell manipulation (Wiklund et al. 2012;Sadhal 2012). In such systems, acoustic waves at MHz frequencies generate acoustic radiation † Email address for correspondence: ashis@iitm.ac.in arXiv: 2602.17081v1 [physics.flu-dyn] 19 Feb 2026 forces (ARF) (Bruus 2012b) in addition to streaming effects (Muller et al. 2012). ARF align suspended particles while streaming, governed by the Stokes drag, tend to disorganize them, particularly at the submicron scale (Bruus 2012b;Muller et al. 2012). Suppression of streaming can greatly benefit the focusing and sorting of submicron particles such as viruses, bacteria, and exosomes for medical diagnostics (Van Assche et al. 2020). On the other hand, enhanced streaming is crucial for overcoming low Reynolds number constraints to improve fluid mixing and pumping (Wiklund et al. 2012). Thus, achieving effective streaming control is critical in acousto-microfluidic systems. Existing streaming suppression methods rely on fluid inhomogeneity (Karlsen et al. 2018) or shape-optimized channels (Bach & Bruus 2020), while streaming enhancement has been achieved using thermal gradients (Qiu et al. 2021); however, these strategies are difficult to implement and offer limited control. Currently, no method enables both suppression and enhancement of acoustic streaming in single-fluid microchannel systems, and none enables its directional reversal. This underscores the need for a simple and effective alternative approach to address a critical gap in the field. Prior studies have examined acoustic streaming in viscoelastic (VE) fluids around a particle (Doinikov et al. 2021a) and in a narrow channel of depth comparable to the boundary layer thickness (Vargas et al. 2022). In this study, we report the enhancement, suppression, and reversal of acoustic streaming in a typical rectangular microchannel by tuning the fluid viscoelasticity. More importantly, we take a profound approach to elucidate the physics underlying acoustic streaming control in terms of relevant timescales, stresses, and viscoelastic shear waves (Stokes 2009;Ferry 1942;Pascal 1989;Ortín 2020).
This study considers a glass-silicon-glass microchannel of width and depth , shown in Fig 1 . The rectangular channel is filled with a viscoelastic fluid characterized by a total viscosity = + , where and denote the polymeric and solvent contributions, respectively, and a relaxation time . Actuation of the piezoelectric transducer generates a standing acoustic wave along the channel width (along -axis), corresponding to a halfwave resonance condition, = 0 /2, where 0 is the wavelength of the standing wave as illustrated in Fig. 1(a). This acoustic excitation perturbs the fluid, inducing spatial variations in the pressure , density , and velocity field (Bruus 2012b). To describe the response of the viscoelastic medium under such excitation, the Oldroyd-B constitutive model (Oldroyd 1950) is used.
To analyze the characteristics of acoustic streaming in a viscoelastic fluid, the formulation begins with the fundamental governing equations, the continuity and momentum equations. The present framework is restricted to dilute polymer solutions for which shear-thinning effects are negligible. Under these conditions, the Oldroyd-B constitutive model (Oldroyd 1950) is employed, as it provides a minimal yet effective description of fluids exhibiting both viscous and elastic responses. This model has been widely used to represent the rheological behavior of dilute polymer solutions and biological fluids in acoustofluidic and related flow configurations (Doinikov et al. 2021a,b;Sujith et al. 2024;Vargas et al. 2022). Accordingly, the governing equations for a viscoelastic fluid are expressed as follows: and
The fluid velocity is expressed as = + , where and are unit vectors along the and directions, and denotes the solvent viscosity. In Eq. 2.2, consistent with Refs. Doinikov et al. (2021a,b);Vargas et al. (2022), for dilute pol
This content is AI-processed based on open access ArXiv data.