Magnetic and optical manipulation of spherical metal-coated Janus particles

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Anna Wang - Spring 2012

Magnetic and optical manipulation of spherical metal-coated Janus particles

This paper is quite long, so this wiki entry will highlight roughly half the results that are mentioned in this paper.


‘Janus’ particles are typically micron-scale colloids with asymmetric surface properties. They are being studied for a variety of reasons – for instance, as macroscopic surfactants (one half is hydrophobic and the other half hydrophilic), as ‘swimmers’ (when one half can catalyse the breakdown of peroxide and hence generate bubbles), and as a means of studying rotation of spherical particles. In this paper, the Janus particles are 10um polystyrene microspheres with a metal hemispherical cap. Their response to electric and magnetic fields is studied for caps of varying geometries.

Results: interaction with magnetic fields

Figure 1. Janus particles rotate under the influence of a magnetic field. Their translational velocity is plotted against speed of rotation.

Janus particles with a hemisphere of ferromagnetic metal are allowed to sediment in DI water towards a glass slide. They are then subjected to an external magnetic field, which causes them to rotate. If the particles were simply rolling, then the relationship between translation and angular frequency is expected to be linear. This, however, was not the case as in Figure 1. The separation between the spheres and the glass slide was then calculated using analysis from Goldman, Cox and Brenner and found to be 120nm, which agrees with an equilibrium position dictated by the competing forces of gravity (particle sedimenting) and electrostatic repulsion of the particle with its image charge (see Image charge) as <math>\epsilon_{water} \approx 81</math> and <math>\epsilon_{glass} \approx 3.9</math>.

Results: interaction with electric fields

Figure 2. Forces involved in trapping a a) transparent b) Janus and c) dot Janus particle.

When particles are much smaller than the wavelength of light, both metal and dielectric particles scatter light similarly. Once the particle becomes roughly the size of the wavelength of light, however, the situation becomes more complicated. One area where this needs to be considered is in optical traps.

The forces involved an optical trap is shown in Figure 2. When the particle is highly asymmetric it is not possible to stably trap it due to the large imbalance in forces. In particular when half the particle is coated with metal, the light will reflect off that half and result in the particle being ejected from the trap. Using optical tweezers to manipulate half-metal Janus particles has hence been a challenge for researchers.

Jenness et al have designed a type of Janus particle that can be stably optically trapped just like a dielectric particle, but can still respond to magnetic traps – the ‘dot Janus’ particle. The methodology for creating such particles is described in their Advanced Materials paper [1]. This means that the particles can be controlled with five degrees of freedom – 3 spatial with the optical trap, and 2 rotational using magnetic fields. The last is constrained as the particle can only be stably trapped if the orientation is such that the metal does not reflect light. This control over so many degrees of freedom is exciting because it allows much more control over directed assembly and increases their potential as a sophisticated biological probe.