Acoustic ‘Distributed Source’ Mixing of Smallest Fluid Volumes

Abstract

Acoustically driven mixing of small fluid volumes is reported. Using surface acoustic waves on a mixer chip, conversion of those into bulk waves, and employing wave guiding effects enables us to distribute a set of "virtual sources" for acoustic streaming over large areas. we demonstrate the applicability of our mixing technique to micro array applications, for mixing of individual wells in a micro titer plate, and other state-of-the-art hybridization assays.Mixing of smallest amounts of fluids is usually a delicate task. For liquids being confined to small volumes, the low Reynold's number [1] is equivalent to an increase of the apparent viscosity. As for mixing applications turbulent streaming which only occurs at high Reynold's numbers (>2000) is advantageous [1], this in turn leads to insufficient mixing in microfluidic systems, as only laminar flow processes are allowed. Hence, the only way for small fluid volumes to effectively mix is driven by diffusion. Here, the smallness of the system is in favour of the diffusion limited time scales, as the respective length scales are equally small. However, for many applications, especially micro array based assays [2], [3], a deliberate and controlled agitation of the fluid under investigation would be of great importance. This is due to the fact that diffusion driven mixing is effective only on a short range. Micro array hybridization, however, demands for transport molecules in the fluid over large distances. Here, agitation is necessary. In micro array applications, usually hundreds or thousands of ‚spots' containing a specific DNA sequence or more generally a specific oligonucleotide are deposited in a matrix like manner, usually on a microscope slide or similar carrier.

Transient Permeabilization of Living Cells: Combining Shear Flow and Acoustofluidic Trapping for the Facilitated Uptake of Molecules

“…This method is suitable for a limited number of adherent cells due to the design limitations of the interdigital transducer and the liquid environment. Even though it was not discussed in their study, the parameters used are known to induce shear flow due to acoustic streaming, especially at high powers [16][17][18][19].…”

Acousto-microfluidics: Transporting microbubble and microparticle arrays in acoustic traps using surface acoustic waves

“…The transverse displacement occurs in the plane normal to the surface of the piezoelectric and can be coupled effectively into a contacting liquid through a transfer of momentum. Recent progress in such SAW based microfluidics has demonstrated mixing of aqueous samples at low Reynolds numbers, [22][23][24] the transport of liquid droplets across dry surfaces, 25 and the ordering of micron-sized particles into arrays in solution. 14,15,26,27 In the latter case, acoustic radiation forces (ARFs) 28 are used to coalesce particles in the nodal positions of a standing pressure wave.…”

Nonlinear Sound Field by Interdigital Transducers in Water

“…1) Subsequently, several studies have been performed on microfluidic applications such as liquid mixing using acoustic streaming by SAW devices. [2][3][4][5] One of the reasons SAW is applied in microfluidic devices is to enable supply of a high driving force to the fluid. The magnitude of the force is basically related to sound intensity and sound absorption coefficient, and the absorption is pronounced at high frequencies.…”