April 14, 2014

Titan Captures Liquid-Crystal Film Complexity

Tiffany Trader

Liquid-crystal displays (familiar to most as LCDs) rely on the light modulating properties of liquid crystals to bring images to life on a wide variety of screens. From computer monitors to televisions to instrumental panels and signage, LCDs are a pervasive element of modern life.

LCDs employ high-tech films, which must be both thin and robust. The problem is that these films degrade over time as liquid-crystal “mesogens,” which make up the films, redistribute to areas of lower energy in a process called dewetting. Eventually the film ruptures.

Recently a team of scientists at Oak Ridge National Laboratory put the lab’s Titan supercomputer – packed with 18,688 CPUs and an equal number of GPUs – to work to better understand the mechanics of this process, as reported on the OLCF website.

Some of the important uses of high-tech films include protecting pills from dissolving too early, keeping metals from corroding, and reducing friction on hard drives. When the films are manufactured using liquid crystals – macromolecules with both rigid and flexible elements – the innovation potential goes through the roof.

The rigid segments support interaction with electric currents, magnetic fields, ambient light and temperature and more. This has led to the material’s wide prevalance in 21st century flat-panel displays. Researchers are actively looking to expand the use of liquid crystal thin films for nanoscale coatings, optical and photovoltaic devices, biosensors, and other innovative applications, but the tendency toward rupturing has stymied progress. By studying the dewetting process more closely, scientists are paving the way for a better generation of films.

For several decades, the prevailing theory held that one of two mechanisms could account for dewetting, and these two mechanisms were mutually exclusive. Then about 10 years ago experiments showed that these two mechanisms did coexist in many cases, as Postdoctoral fellow Trung Nguyen of Oak Ridge National Laboratory (ORNL) explains. Nguyen, who was coprincipal investigator on the project with W. Michael Brown (then at ORNL, but now working at Intel), ran large-scale molecular dynamics simulations on ORNL’s Titan supercomputer detailing the beginning stages of ruptures forming on thin films on a solid substrate. The work appears as the cover story in the March 21, 2014, print edition of Nanoscale, a journal of the Royal Society of Chemistry.

“This study examined a somewhat controversial argument about the mechanism of the dewetting in the thin films,” stated Nguyen.

The two mechanisms thought to be responsible for the dewetting are thermal nucleation, a heat-mediated cause, and spinodal dewetting, a movement-induced cause. Theoretical models posited decades ago asserted that one or the other would be responsible for dewetting thin film, depending on its initial thickness. The simulation validated that the two mechanisms do coexist, although one does predominate depending on the thickness of the film – with thermal nucleation being more prominent in thicker films and spinodal dewetting more common in thinner films.

The impetus for the ruptures is the liquid-crystal molecules striving to recover lower-energy states. While still in the research stages, it is thought that this finding may boost innovation in using thin films for applications such as energy production, biochemical detection, and mechanical lubrication. The research was facilitated by a 2013 Titan Early Science program allocation of supercomputing time at the Oak Ridge Leadership Computing Facility. Nguyen’s team went through the ORNL’s Center for Accelerated Applications Readiness (CAAR) program, which gives early access to cutting-edge resources for codes that can take advantage of graphics processing units (GPUs) at scale. Under the CAAR program, Brown reworked the LAMMPS molecular dynamics code to leverage a large number of GPUs.

Titan, the most powerful US supercomputer and the world’s second fastest, has a max theoretical computing speed of 27 petaflops and a LINPACK measured at 17.59 petaflops. The Titan Cray XK7 system is also the first major supercomputing system to utilize a hybrid architecture using both conventional 16-core AMD Opteron CPUs plus NVIDIA Tesla K20 GPU parts.

The researchers utilized Titan to simulate 26 million mesogens on a substrate micrometers in length and width, employing 18 million core hours and harnessing up to 4,900 of Titan’s nodes. The study lasted three months, but would have taken about two years without the acceleration of Titan’s GPUs.

“We’re using LAMMPS with GPU acceleration so that the speedup will be seven times relative to a comparable CPU-only architecture – for example, the Cray XE6. If someone wants to rerun the simulations without a GPU, they have to be seven times slower,” Nguyen explained. “The dewetting problems are excellent candidates to use Titan for because we need to use big systems to capture the complexity of the dewetting origin of liquid-crystal thin films, both microscopically and macroscopically.”

This is the first study to simulate liquid-crystal thin films at experimental length- and timescales and also the first to relate the dewetting process to the molecular-level driving force, which causes the molecules to break up.

The Nanoscale paper was also authored by postdoctoral fellow Jan-Michael Carrillo, who worked on the simulation model, and computational scientist Michael Matheson, who developed the software for the analysis and visualization work.

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