New research on liquid crystals
May 14, 2014
Liquid crystals can be used to detect breast cancer. Cancerous cells are warmer than normal cells, and doctors can hold a film containing liquid crystals over the skin to detect cancer cells. This is because the film responds differently to temperature. It is only recently that this technology was made portable.
A faculty member from Delaware Valley College’s chemistry department, Dr. Edward Sambriski, recently published research that sheds light on how liquid crystals move and for how long they can support certain patterns in devices that use them.
Looking at how molecules in a liquid crystal move in time helps to understand the exciting technology that these materials can offer. What’s new here is the type of information that comes from the computational studies that follow how these molecules move in space and time.
Dr. Sambriski coauthored, “Phase equilibria, fluid structure, and diffusivity of a discotic liquid crystals” in Soft Matter, a scientific journal published by the Royal Society of Chemistry. He collaborated with two physicists from the Autonomous Metropolitan University (Iztapalapa) in Mexico City on the project.
Liquid crystals flow like a normal liquid, but unlike a normal liquid, they can flow with structural order. Factors like temperature, pressure, and light have an impact on how these molecules organize in the liquid. Most likely, you’ve encountered these materials before in a liquid crystal display, or LCD.
Dr. Sambriski studied discotic liquid crystals, which are made from disc-shaped molecules. Discotic liquid crystals have the ability to form columns under certain conditions, and the research team he was part of wanted to find out how these materials organize on a molecular scale.
Many flexible screens and photovoltaic devices, such as solar panels, make use of discotic liquid crystals. Recently, curved television screens have been developed with discotic liquid crystals to overcome screen glare and to remove distortion from any viewing angle. Their applications go on, so understanding how discotic liquid crystals behave can shed light on new developments and improvements.
Because many liquid crystal systems are carbon-based, devices produced from them tend to be cheaper to manufacture (because carbon is readily available) and can overcome some of the environmental impact associated with their inorganic (metal-based) counterparts.
The discotic liquid crystal can organize in a hexagonal pattern at high temperatures and high pressures, or in a rectangular pattern at low temperatures and low pressures. The rectangular arrangement forms into columns, making it less mobile and useful in applications that require transfer of electrical current.
As soon as a column of liquid crystals forms, limited motion dominates in the sample, but molecules can occasionally swap spots from one column of liquid to another. Rattling motion is quite common in columns, but there is also a net drift of the liquid along the column. Molecules can undergo string-like motion when they move together, leading to a “breathing” motion in the sample.
“By mapping out how these materials relax or become stable, we can understand and predict how they behave over time,” said Dr. Sambriski. “The lifetime and stability of devices that use liquid crystals are issues that must be considered in their development.”
Calculations used in the study are costly because systems have to be monitored on a large scale to get accurate results.
“The dynamics differ depending on the conditions you have, and certain features can only be observed when you wait long enough, or probe the material on a sufficiently large spatial scale” said Dr. Sambriski. “Computer simulations allow you to map out how liquid crystals evolve with time.”
The team used supercomputers to carry out the research. The computer simulations allow the team to change experimental conditions at will and better understand how liquid crystals behave over time, at different temperatures and pressures.
“In a way, computer simulations provide the individual frames that, when put together, make a movie showcasing what molecules do… something that is still not possible to see in detail with instruments,” said Dr. Sambriski of the process for understanding the molecular dynamics of discotic liquid crystals.