Manipulation of Non-Magnetic Materials in Ferrofluid Containing Media
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Manipulation of Non-Magnetic Materials in Ferrofluid Containing Media Derek Halverson, Ben Yellen, Gary Friedman Drexel University e-mail: [email protected] Abstract A novel method is proposed whereby non-magnetic objects can be moved along a surface at the microscale and nanoscale. It uses a negative magnetophoretic force, explained in the caption for figure one, on the non-magnetic objects which results from stabilized 10nm diameter iron oxide particles (ferrofluid) being attracted to regions of field maxima around magnetic islands on a surface, which pushes the non-magnetic objects to regions of field minima. By varying an external magnetic field we can control where these minima are and thus control how objects will position themselves with static fields and by using rotating time varying fields we can control how they move across the surface. This method does not require the objects to be initially in contact with the surface, as they will be pulled down to the surface from solution. While this paper deals with beads, any arbitrarily shaped object should be manipuable using this method. Additionally, while we address non-magnetic objects in this work similar methods could easily manipulate objects that are magnetic. Introduction While robotic assembly has been very effective in manufacture of systems consisting of many different components on the macroscopic scale, it is difficult to extend this technique to micro- and nano-scale. Techniques inspired by self-assembly of structures in natural settings appear to be a promising alternative. Massively parallel assembly methods based on laser tweezing, electric and ultrasonic fields, gravitation, flow, morphological templates and chemical recognition have been explored. Among all these different assembly methods few offer the possibility of programmable assembly and manipulation of micro- or nano-components. Dielelectrophoretic manipulation is one of the methods particularly suitable to programmable manipulation of components. However, methods involving charged surfaces [1] place restrictions on the chemistry that can be used with them. A novel method of massively parallel programmable manipulation and assembly of nonmagnetic components such as colloidal particles will be discussed in this paper. In this method, non-magnetic components are dispersed inside ferrofluid. A pattern of magnetic field maxima and minima on a surface is created using a pattern of magnetized thin Co islands and an external uniform magnetic field. Non-magnetic particles are attracted to regions of magnetic field minima through a process that can be called negative magnetophoresis forming highly ordered patterns. This attraction is explained further in figure one. The force of this attraction will be proportional to the gradients in the fields. Therefore the force will be strong even for very small islands because while their magnetic moment will be smaller the gradients will still be strong. Note that the field minima will always be near the surface of the islands and will usually be f
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