Oberti Stefano, Neild Adrian, Möller Dirk, Dual Jürg
Institute of Mechanical Systems, Department of Mechanical and Process Engineering, IMES, ETH Zurich, CH-8092 Zurich, Switzerland.
Ultrasonics. 2008 Nov;48(6-7):529-36. doi: 10.1016/j.ultras.2008.06.004. Epub 2008 Jun 13.
The use of acoustic radiation forces for the manipulation and positioning of micrometer sized particles has shown to be a promising approach. Resonant excitation of a system containing a particle laden fluid filled cavity, can (depending on the mode excited) result in positioning of the particles in parallel lines (1-D) or distinct clumps in a grid formation (2-D) due to the high amplitude standing pressure fields that arise in the fluid. In a broader context, the alignment of particles using acoustic forces can be used to assist manipulation processes which utilise an external mechanical tool, for instance a microgripper. In such a system, particles can be removed sequentially from a line formed by acoustic forces within a microfluidic channel, hence allowing a degree of automation. In order to fully automate the gripping process, the particles must be confined to a repeatable and accurate location in two dimensions (assuming that in the third dimension they sit on the lower surface of the channel). Only in this way it is possible to remove subsequent particles by simply bringing the gripper to a known location and activating its fingers. This combined use of acoustic forces and mechanical gripping requires that one extremity of the channel is open. However, the presence of the liquid-air interface which occurs at this opening, causes the standing pressure field to decay to zero towards the opening. In a volume of liquid in proximity to the interface positioning of particles by acoustic forces is therefore no longer possible. In addition, the longitudinal gradient of the field can cause a drift of particles towards the longitudinal center of the channel at some frequencies, undesirably moving them further away from the interface, and so further from the gripper. As a solution the use of microfluidic flow induced drag forces in addition to the acoustic force potential has been investigated.
利用声辐射力对微米级粒子进行操纵和定位已被证明是一种很有前景的方法。包含充满载有粒子流体的腔体的系统的共振激发，（取决于所激发的模式）由于流体中出现的高振幅驻压场，可导致粒子排列成平行线（一维）或形成网格状的不同团块（二维）。从更广泛的背景来看，利用声学力使粒子排列可用于辅助使用外部机械工具（例如微夹钳）的操纵过程。在这样的系统中，粒子可以从微流体通道内由声学力形成的线中依次移除，从而实现一定程度的自动化。为了使抓取过程完全自动化，粒子必须在二维空间中被限制在一个可重复且精确的位置（假设在第三维中它们位于通道的下表面）。只有这样，才有可能通过简单地将夹钳移动到已知位置并激活其夹指来移除后续粒子。声学力和机械抓取的这种联合使用要求通道的一端是开放的。然而，在这个开口处出现的液 - 气界面会导致驻压场朝着开口处衰减至零。因此，在靠近界面的液体体积中，利用声学力对粒子进行定位不再可能。此外，场的纵向梯度在某些频率下会导致粒子朝着通道的纵向中心漂移，不希望地使它们远离界面，从而进一步远离夹钳。作为一种解决方案，除了声学力势之外，还研究了利用微流体流动诱导的拖曳力。
