Selective Trapping and Manipulation of Microscale Objects Using Mobile Microvortices Tristan Petit, † Li Zhang,* Kathrin E. Peyer, Bradley E. Kratochvil, and Bradley J. Nelson Institute of Robotics and Intelligent Systems, ETH Zurich, CH-8092 Zurich, Switzerland * S Supporting Information ABSTRACT: Controlled manipulation of individual micro- and nanoscale objects requires the use of trapping forces that can be focused and translated with high spatial and time resolution. We report a new strategy that uses the flow of mobile microvortices to trap and manipulate single objects in fluid with essentially no restrictions on their material properties. Fluidic trapping forces are generated toward the center of microvortices formed by magnetic microactuators, that is, rotating nanowire or self-assembled microbeads, actuated by a weak rotating magnetic field (|B|< 5 mT). We demonstrate precise manipulation of single microspheres and microorganisms near a solid surface in water. KEYWORDS: Fluidic trapping, nanowire, microvortex, low Reynolds number flow, noncontact manipulation O ptical tweezers, 1-3 magnetic tweezers, 4-6 and dielectro- phoresis 7,8 are commonly used for the manipulation of individual microscale objects such as microspheres, 9 cells, or bacteria. 10,11 A limitation of these techniques is that high intensity lasers, physical attachment to magnetic objects, or strong electrical fields cannot be used with many biological samples. 11,12 Several alternative methods based on near field photonics, 13,14 electrostatic, 15 electokinetic, 16,17 or acoustic traps 18 have been proposed to circumvent these limitations but only offer static trapping possibilities. Single-cell manipu- lation with a high throughput can be achieved using microfluidics, 19 however the fluid flow is generally controlled by geometrical means, such as channels or pillars in “lab-on-a- chip” systems, 20-22 which limits its versatility for manipulation tasks. While static trapping of micro-objects has been demonstrated in recirculating flows generated by different device geometries, 23-25 dynamically manipulating a single micro-object with high spatial resolution is not possible with these techniques. We have created a new approach for precisely manipulating micro-objects using mobile microvortices at low-Reynolds numbers (10 -1 to 10 -4 ) generated by the rotation of magnetic microactuators such as nickel nanowires or self-assembled magnetic bead doublets in fluid. A trapping force is locally induced by the flow velocity gradient toward the center of the microvortex. The amplitude and position of the microvortex can be precisely controlled to selectively trap and transport individual micro-objects near a solid surface. Only a very weak rotating uniform magnetic field is required as an energy source (|B|< 5 mT). This makes the technique well adapted for manipulating biological samples over a large working volume and can be incorporated into most existing microfluidic systems. When a microactuator rotates around its geometrical center, a localized shear flow is formed between its two distal ends, 26 as shown in the simulation of the tangential velocity in Figure 1a. The model uses the low Reynolds number assumption and is based on the method of fundamental solutions 27 (see Supporting Information). The model shows the tangential flow near a 13 μm long nanowire rotating at 95 Hz, which corresponds to a rotational speed of 5700 rpm in an unbounded fluid. The flow velocity and the shear rate decrease rapidly with distance from the nanowire. Averaged over one complete rotation of the nanowire, the amplitude of the tangential flow profiles in planes normal to the rotational axis exhibits a radial symmetry. The simulation indicates that a local minimal flow velocity exists at the center of the nanowire, whereas maximal flow velocities occur above its ends. Therefore, the averaged tangential flow resembles a micro- vortex centered on the rotational axis of the nanowire, as shown in Figure 1b. The shear rate in planes normal to the rotational axis is directly tuned by the rotational speed of the nanowire, which in turn is controlled by the frequency and strength of the rotating magnetic field. Such a microvortex is experimentally demonstrated by rotating a 13 μm long nickel nanowire with a diameter of ca. 200 nm horizontally above a flat silicon surface in sync with a rotating magnetic field of 3 mT. The uniform magnetic field was generated by a three-axis Helmholtz coil setup. Further experimental details are available in the Supporting Informa- tion. To observe the hydrodynamic interaction between the microvortex and a microobject, a 6 μm polystyrene micro- Received: September 17, 2011 Revised: November 18, 2011 Published: November 23, 2011 Letter pubs.acs.org/NanoLett © 2011 American Chemical Society 156 dx.doi.org/10.1021/nl2032487 | Nano Lett. 2012, 12, 156-160