Abstract

1. Search for an oil drop formation by phase separating from a liquid crystal mixture Erik Walker; Charles Rosenblatt, Department of Physics Case Western Reserve University Abstract - When an oil and liquid crystal mixture are droplets appear, coalesce and line up along the director. We searched for ways to control the direction of the oil drop formation. By cooling quickly from high temperatures, phase separation may occur. Oil the oil and liquid crystal mixture to isotropic and cooling it slowly to the mixtures nematic phase, we can cool it quickly and observe the phase separation of the oil. We made cells of rubbed PVA coated glass filled with an oil/liquid crystal mixture. We hoped to control the direction and formation of these oil drops using different rubbing strengths, different oil/liquid crystal mixtures, and different substrates. We saw oil drops form along the direction of the rubbing but they were not consistent. • Methods • A 5% PVA solution is made with water. To make the cells, clean glass is used. To add the PVA film to the glass, we use spin coating. We also use this technique for coating the glass with other substrates. We control the thickness of the film by the rpm and the duration. Nine seconds at 1500 rpm works well. Then, the glass coated with the substrate is baked to remove the solvent. We then rub the PVA coated glass substrate with a velvet rubbing machine three times at 500 rpm. This is a variable that helps determine how well the oil drops will align. Too hard and scratches will appear on the glass, and too light and the oil drops will not form chains. After the PVA coated glass has been rubbed, the cell can be made. To make the cell, twenty five micron thick spacers are used between the PVA coated glass and a bare glass plate or any other substrate that is being used. To make the oil/liquid crystal mixture, 2.5% silicone oil in E7 LC mixture (Merck) is used. • Both the cell and oil/liquid crystal mixture are heated to isotropic. The oil/liquid crystal mixture is then added to the cell using capillary action. The cell is then cooled to its nematic phase (around 52*C). It is kept there for about two minutes and then placed on cool metal surface at room temperature. This process of heating and cooling can be repeated a few times. The cell can be observed after a couple minutes Results - Although we were able to replicate the methods of the original experiment along with acquiring samples from the original experiment, we were unable to achieve the expected results. The droplet chains that were produced in this experiment were not long enough to control. I saw 20 droplet chains dispersed throughout a cell while we were expecting to see chains of up to a couple hundred, all equally spaced and well aligned with each other. Figure 3 You can see in figure 3, the direction of alignment(1). Also note the droplets(2) and some formations of chains(3) Introduction -Some liquid crystals have a single phase at high temperatures and two at lower temperatures. Phase separation may occur when an oil/liquid crystal mixture is heated to isotropic and then quenched. In a binary mixture, droplets follow Brownian motion and coalesce as time elapses. When there is a liquid crystal mixed in, then the droplets follow the liquid crystal orientation over time. Ideally, the mixture would self-align into well aligned chains made of monodisperse colloidal droplets. The direction of alignment is controlled by the unit vector field called the director. As droplets form, they provoke distortions in the director and it produces elastic forces between the droplets. There is also an anchoring energy that comes from the interaction among the droplets and the liquid crystals. When there is a strong anchoring force, there is a well defined director, which produces well defined droplet chains. On the other hand, the chains are indistinct when there is a weak anchoring force. These oil drops show a dipole configuration which leads to the droplets forming chains. Figure 2 Figure 2-The chart for the liquid crystal phases. There isotropic, nematic, and isotropic and nematic phases Conclusion and Forward -There is clearly is a difference between our samples and those of Loudet,the original experimenter, with whom we have been in contact multiple times. Professor Rosenblatt will visit Loudet in Bordeaux on May 22 in order to figure out what is going on. Figure 2 Acknowledgements -Thank you to Professor Rosenblatt for all his help along with Tim, Valintin, and Daeseung . In figure 1, this is what the nematic phase of the liquid crystal looks like. • Resources • - Loudet, Jean-Christophe. Colloidal inclusions in liquid crystals: Phase separation mechanisms and some dynamical aspects. Liquid Crystals Today. 14 (2005): 1-14 • Loudet, Jean-Christophe. Colloidal ordering from phase separation in a liquid crystalline continuous phase. Letters to Nature. 407 (2000): 611-613 • Order Parameter Fields. http://www.lassp.cornell.edu/sethna/LiquidCrystals/BluePhase/fig/Nematic_med.gif