Novel experimental concept provides structural insights

Scattering pattern of a liquid crystal dispersion as observed during an oscillatory shear flow experiment

Scattering pattern of a liquid crystal dispersion as observed during an oscillatory shear flow experiment. (Photo Credit: Pavlik Lettinga)

From blood vessels to cement mixers – information about how substances flow is key knowledge for a wide variety of systems. The viscosity and elasticity of fluids are routinely investigated in food, cosmetics, oil and other industries. However,companies usually measure these properties with little understanding of what is going on inside the sample. Scientists from Forschungszentrum Jülich, Utrecht University, and Deutsches Elektronen-Synchrotron (DESY) have now chosen a novel approach to flow studies. With a unique setup at DESY’s PETRA III light source, the researchers monitored changes of the viscoelastic properties of liquid crystals under stress while, at the same time, followed the inner structure of the sample.

The German-Dutch research team exposed a preparation of liquid crystals to stress by rotating the sample back and forth. Taking ten X-ray images per second with PETRA’s bright X-ray beam, the scientists simultaneously “filmed” the sample’s structural response. The study, which was published online today by the journal Physical Review Letters, may transform how flow studies will be approached in the future.

A rheometer with X-ray vision

Industrial businesses use so-called rheometers to analyze the flow properties of their products. These devices monitor a sample’s motion in response to an external force. Companies use rheometers, for instance, to adjust how yoghurt feels on the tongue or how body lotion feels on the skin. “Yet, a rheometer alone cannot tell you anything about the inner structure of a sample,” says DESY scientist Bernd Struth, one of the study’s authors. “It would be of great interest if we could understand, in detail, how the flow behavior is determined by the microstructures inside the substance and, hence, how we can control the flow properties.”
For that reason, Struth (together with Thermo Fisher Scientific Inc.) developed a new type of rheometer that can be utilized at powerful X-ray light sources such as PETRA III. One of its unique features is a specially developed optical element. It allows the X-ray beam to travel vertically through the setup although it emerges from the light source in the horizontal plane. “This setup is one of a kind, and it enables us to do completely new science,” says Struth. “We can study how a sample flows and, at the same time, examine with the X-ray beam how the molecules arrange in it.”
At the PETRA beamline P10, scientists load their sample between two horizontal plates. One of the plates remains fixed while the other rotates back and forth, causing so-called shear stress. The external force deforms the sample, and the researchers analyze if the sample responds like a fluid or more like an elastic spring. Simultaneously, a powerful X-ray beam travels through the setup. From the scattering pattern, which the X-rays form on a detector, the scientists calculate the orientation of particles inside their samples at each point in time.

Unexpected flow behavior of liquid crystals

The researchers are particularly interested in understanding how liquid crystals can be switched between different flow behaviors. “Drilling mud, for example, which is used in the process of pumping oil through boreholes, can be switched from a state in which it flows to a state in which its motion freezes,” says Pavlik Lettinga, the study’s first author from the research center in Jülich. “We studied the mineral gibbsite because it is a good model system for substances such as drilling mud.”
When in rest, platelets of gibbsite are known to settle with their flat surfaces parallel to the walls of the container they are in. The platelets with a typical diametre of 250 nanometres can easily slide on top of one another as layers of liquid while maintaining order like a crystal, hence the term “liquid crystal”. But what happens when the platelets are forced out of their comfortable position by applying shear flow? Do they flow like a liquid or are they rather elastic like a solid? Shear flow, where a moving plate moves back and forth between two positions, is an ideal method to probe the dual solid-liquid behavior of gibbsite platelets.
The researchers found that the response of the platelet structure depends on the amplitude of oscillation and, hence, the amount of deformation. If the amplitude is large enough, the platelets tilt up from their initial position and flip over like a playing card. This happens twice per oscillation cycle – at the points of flow reversal. At those points, the sample is elastic, whereas between the flips the platelets slide on top of one another like a fluid.
For small amplitudes, in contrast, the platelets tilt up without flipping over. The playing card gets only lifted. When the flow is reversed, the platelets return to their original position as if the card is put back down. For small amplitudes, the platelets respond like an ideal solid and are elastic over the entire oscillation cycle.
“With our method, we can precisely determine why the sample flows and why it becomes elastic,” says Lettinga. “Similar transitions in flow behaviour may occur in other viscoelastic substances as well. However, they may have gone unnoticed because people have not looked into their samples with x-rays before.”
The approach to flow studies may change in the near future. “We are now able to see how the transition from elastic to viscous behavior relates to changes of the microstructure inside a sample,” Struth says. “With our data as input, theoreticians can start developing models for such transitions.”Accurate models would be an important step towards predicting and tailoring the flow properties of various materials.
One of the best known uses of liquid crystals is in displays (LCD, liquid crystal display). In LCDs, the optical properties of liquid crystals are switched electrically. The researchers, in comparison, switched liquid crystal flow properties mechanically. It would be exciting to investigate if their “mechanical switch” can also be applied to LCDs.
Reference: The non-linear behavior of nematic platelet dispersions in shear flow; M.P. Lettinga, P. Holmqvist, P. Ballesta, S. Rogers, D. Kleshchanok, and B. Struth; "Physical Review Letters" (DOI: 10.1103/PhysRevLett.109.246001).

Video of the changing scattering pattern when applying an oscillation with strain amplitude of 12.8 and frequency of 0.04 Hz. Whenever the gibbsite platelets flip over, the researchers observe peaks in the scattering pattern. For large oscillation amplitudes, this happens exactly twice per cycle - when the flow direction changes.

Pavlik Lettinga and Bernd Struth are co-organizers of an “In Situ Rheology” workshop to be held at DESY on January 24-25, 2013.