Optically Anisotropic Colloids of Controllable Shape
Wiki entry by Emily Gehrels, Fall 2012 Based on the article: Fernández-Nieves, A., Cristobal, G., Garcés-Chávez, V., Spalding, G.C., Dholakia, K., Weitz, D.A. (2005). "Optically Anisotropic Colloids of Controllable Shape." Adv. Mater., vol. 17 no. 6 pp. 680-684.
A colloidal suspension occurs when identical, small (1-1000 nm), solid objects are suspended in a liquid. Objects are optically anisotropic if they react differently to light of different polarizations. This is also known as birefringence and results from a material having different refractive index depending on the polarization of incoming light. This paper deals with monodisperse optically anisotropic colloids of different shapes. Optically anisotropic colloids are of interest because of their ability to be manipulated with external light such as that used in optical tweezers.
Birefringent Liquid Crystal Colloids Using Microfluidics
The Weitz group here uses microfluidics to create monodisperse drops of liquid-crystal in water. They do this by pushing a liquid crystal emulsion out of a small capillary (inner diameter a few micrometers) that is surrounded by flowing water. When the drop being pushed out of the capillary reaches a critical size, the viscous drag on it from the flowing water becomes larger than the surface tension holding it to the capillary and it breaks off from the capillary. Since the force balance that leads to the drop breaking off will happen at the same drop radius each time, the droplets formed are monodisperse (all have very similar size). Different surfactants can be combined with the flowing water to keep the drops from sticking together once they are formed and can also alter the configuration of the liquid crystal within the droplet. In the case of this paper the surfactant poly(vinyl alcohol) (PVA) is used to give the liquid crystal droplets bipolar with two boojums at opposing sides of the droplet. This is what causes the birefringence in the colloids.
Spherical Birefringent Colloids
To make solid birefringent particles the liquid crystal emulsion used is is a combination of UV-reactive liquid-crystal monomer and a photoinitiator. This particular combination is slightly inconvenient for use in microfluidics because it needs to be kept at 40˚C and is highly viscous, so very high pressures have to be used in the device. To make it easier to work with, chloroform is mixed with the photoinitiator, which decreases the viscosity and allows the liquid crystal to be worked with at room temperature. After the drops are formed, they are left to sit for ~12 hours so that the chloroform evaporates out of the droplets. The addition of the chloroform also allows the size of the drops controlled since the drops will still break off at the same size as mentioned in the previous section, but will now shrink by a known amount as the chloroform evaporates. By controlling the concentration of chloroform in the liquid crystal emulsion you can therefore control the final size of the droplets between about 2 and 30 microns with a polydispercity of less than 10 percent. Once the chloroform has evaporated from the droplets, they are exposed to UV light for 10 minutes at 60˚C to polymerize the liquid crystal emulsion.
Bipolar Birefringent Colloids
Using a similar method, bipolar molecules can be made. In this case the droplets are made using the same microfluidic method. The PVA content of the water in which the droplets are suspended is increased and a few milliliters of this solution is placed on a substrate and spread into a thin film. The water is allowed to evaportate from the solution leaving a PVA polymer matrix embedded with liquid crystal droplets. This film was then peeled off and stretched along one axis. Once the stretching was completed, the film was UV polymerized and then redispersed in water, dissolving the PVA film.
These bipolar colloidal particles can be used for several applications. The most promising is to use the particles as microstirers. These elliptical particles can be trapped using optical tweezers because of their optical anisotropy. If the optical tweezers consist of circularly polarized light, the particle will rotate with the rotating electric field of the light. The paper shows that there is a linear relationship between the power of the laser used and the particle-rotation frequency. This linear dependence arises from a balance between the torque from the electric field and the viscous torque, which is proportional to the angular speed of rotation. This microstirer can be used not only to stir liquids of very small volumes, but can also be used as a micrometer scaled rheometer to measure the viscosity of a fluid.