A “smart fluid” you can reconfigure with temperature

(Press-News.org) Imagine a “smart fluid” whose internal structure can be rearranged just by changing temperature.

In a new study in Matter, researchers report a way to overcome a long-standing limitation in a class of “smart fluids” called nematic liquid crystal microcolloids, allowing for reconfigurable self-assembly of micrometer-sized particles dispersed in a nematic liquid crystal host.

The persistent challenge has been that conventional microparticles can induce strong distortions and topological defects in the liquid crystal, which in turn can drive irreversible sticking and clumping, making it difficult to reach equilibrated, reconfigurable states.

Here, the team developed porous, rod-shaped silica microrods with “slippery” surface treatment that can form dense dispersions that remain fluid-like while reorganizing with temperature.

“Materials like this could one day support reconfigurable optical components, potentially changing how screens control light, how photonic chips process information, or how biomedical sensors detect and report conditions,” explains corresponding author Ivan Smalyukh, director of Hiroshima University’s International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM²) satellite at University of Colorado Boulder and Professor of Physics at CU Boulder.

A major thrust of Smalyukh’s work and of WPI-SKCM² is to develop “meta matter,” materials whose behavior is engineered through the geometry and orientation of their internal building blocks, not only through chemical composition.

What are nematic liquid crystal microcolloids? Milk, with microscopic fat droplets suspended in water, is a classic example of a colloid. These small particles dispersed in a liquid scatter light, which is why milk appears white. But in milk, the droplets don’t organize into any pattern.  On average, the suspension looks the same in every direction, a property physicists call rotational symmetry.

To create structure, scientists can place colloids inside a nematic liquid crystal, the same general class of material used in LCD screens. Nematic liquid crystals break rotational symmetry by developing orientational order. Their nanometer-scale rod-like molecules tend to align with one another, so the liquid has a kind of dynamic “grain,” in which the molecules have a shared direction but are not locked into a rigid lattice like in a crystal. In the late 1990s, researchers reported that when colloids are placed in this directional fluid, the colloids will assemble according to the liquid crystal.

Until now, a major obstacle to making stable and thus reconfigurable nematic liquid crystal microcolloids is that the colloidal particles often force nearby molecules to point in a particular way, an effect called surface anchoring. Strong anchoring can create large distortions and defects in the liquid crystal’s orientation pattern. Those distortions can attract other particles and promote sticking and irreversible aggregation, preventing equilibration and reconfiguration.

Developing an improved colloid In this study, the team developed an improved colloid.  They are silica microrods about 2–3 μm long and 200–300 nm in diameter whose surfaces are etched to be porous and then treated with a perfluorocarbon coating.

Because even subtle surface-treatment details strongly affect how liquid crystal molecules anchor to the microrods, optimizing the treatment required dedicated, meticulous effort from Souvik Ghosh, PhD, the study’s first author who was a research associate at CU Boulder at the time of the work.

When dispersed in a common nematic liquid crystal called 5CB, these porous, slippery microrods exhibit reduced effective surface anchoring, meaning the liquid crystal molecules at the surface can more easily deviate from the preferred orientation. As a result, the rods produce only weak distortions in the surrounding liquid crystal, which helps them remain dispersed and mobile rather than irreversibly aggregating.

Temperature reconfigures colloidal order Once the authors had created a stable, dense dispersion of porous microrods in a nematic liquid crystal, they tested what kinds of behavior this hybrid material could produce. They tracked how rod orientation and collective phase behavior change with temperature and rod concentration. As temperature changes, the rods reorient, and in dense samples the suspension switches between distinct patterns, or phases. This includes several unexpected low-symmetry phases, unusual ordered states with more than one distinguished direction of alignment, rather than a single nematic grain direction.

“Low-symmetry liquid crystals are difficult to achieve,” notes Smalyukh. This hybrid liquid crystal can support more complex organization than a standard nematic, allowing the group to probe a fundamental question in condensed-matter physics regarding the types of order that can coexist with fluidity. 

They used a tensorial Landau de Gennes model, with coupled alignment tensors for the molecular host and the colloids.  It explains how host-colloid coupling can stabilize low-symmetry hybrid phases with multiple distinguished directions, rather than limiting the system to the ordinary uniaxial nematic order available to each component on its own. 

Temperature tunes the effective coupling because heating changes the preferred alignment of the liquid crystal at the rod surface, and this change in anchoring is sufficient to drive the rods to rotate into a new equilibrium orientation. 

This framework emerged from joint discussions between Lech Longa, Professor of Theoretical Physics at Jagiellonian University and WPI-SKCM² Community Member, and Smalyukh at Hiroshima University in early 2024.  Longa was an invited guest lecturer for the WPI-SKCM² winter school program, which provides graduate students and postdocs advanced and interdisciplinary training related to knot topology and chirality. 

Meta materials and model systems ​​“Liquid crystals used in displays, and membranes in biological cells, are examples of how combinations of order and fluidity can enable utility and functionality. Low-symmetry liquid crystals may offer even greater opportunities,” says Smalyukh.

He adds, “including the potential to host new types of solitons and knotted structures,” which connects to one of WPI-SKCM²’s founding visions of investigating the potential for knotted and chiral structures to serve as building blocks for pre-designed meta matter.

WPI-SKCM² is one of 18 research centers supported by Japan’s World Premier International Research Center Initiative (WPI), a Japanese government-funded program that promotes long-term, interdisciplinary research in an international research environment.

While these nematic liquid crystal microcolloids may have future applications in soft-matter-based technologies, such as electro-optics, photonics, biomedical sensing, they also may contribute to fundamental science by expanding the toolkit of colloids as model systems.

Colloids already act as “visible analogues” of atoms and molecules, allowing researchers to directly watch how building blocks organize into phases, transform, and reorganize under stimuli in ways that are hard to observe directly at atomic scales. The researchers propose that the low-symmetry phases found in these nematic liquid crystal colloids could be used as model systems for topological solitons and singular defects.

Such model systems can help reveal how these particle-like features behave and persist, offering insights that reach beyond soft matter to magnetism, superconductors, and particle physics.

 

 

 

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