This image sequence shows the reversible shape transformation of a sphere made from liquid crystal rubber; at 20°C, it adopts a folded shape similar to spherical pancakes but turns into a smooth sphere at 60°C, and vice versa. The characteristic colors on the sphere reflect the anti-ordered microscale arrangement of molecules. Image: University of Luxembourg.In liquid crystals, molecules automatically arrange themselves in an ordered fashion. But researchers from the University of Luxembourg have now discovered a method for producing an anti-ordered state, which could lead to novel material properties and potentially new technical applications, such as artificial muscles for soft robotics. The researchers report their findings in a paper in Science Advances.
The research team of Jan Lagerwall at the University of Luxembourg studies the characteristics of liquid crystals, which can be found in many places, from cell membranes in the body to displays in electronic devices. Liquid crystals combine liquid-like mobility and flexibility with long-range order of its molecules, which is a typical feature of solid crystals. This gives rise to remarkable properties that render liquid crystals so versatile they are chosen for carrying out vital functions by both nature and billion-dollar companies.
Many of a material's properties are dictated by the way its molecules are arranged. Since the late 1930s, physicists have used a mathematical model to describe the molecular order in liquid crystals: the so-called order parameter assigns a number that reflects how well ordered the molecules are. This model uses a positive range to describe the liquid crystals that we are used to. It can also assign a negative range to describe an ‘anti-ordered’ state, where the molecules avoid a certain direction rather than align along it.
So far, this negative range has remained strictly hypothetical, as no liquid crystal has ever adopted an anti-ordered state in practice. The standard theories for liquid crystals suggest that such a state is possible, but would not be stable.
"You can compare this to a slide that has a very light bump in the middle," explains Lagerwall. "You may slow down when you reach the bump – in our case the unstable anti-ordered state – but not enough so you stop, and therefore you will go down all the way to the stable state, the global energy minimum, where you inevitably end up with positive order. If you could manage to stop the ride at the bump, a negative range would be possible."
This is exactly what Venkata Jampani, the main author of the paper, and his co-workers have now managed to achieve, for the first time, in their study. "The trick for preventing the system from reaching the global energy minimum is to gently polymerize it into a loosely connected network while it is dissolved in a normal liquid solvent," says Jampani. "This network is then stretched in all directions within a plane, or compressed along a single direction perpendicular to the plane, such that the molecules forming the network align into the plane, but without any particular direction in that plane."
As the solvent is evaporated the liquid crystal phase forms and, due to the peculiar in-plane stretching of the network, it is forced to adopt the negative-order parameter state, where the molecules avoid the direction normal to the plane. "This liquid crystal has no choice but to settle with the secondary energy minimum, since the global energy minimum is made inaccessible by the network," adds Lagerwall.
When the network is strengthened by a second round of polymerization, its behavior as a function of temperature can be studied. "Liquid crystal networks are fascinating for positive as well as negative order parameter, because the ordering – or anti-ordering – in combination with the polymer network allows it to spontaneously change its shape in response to temperature changes. The liquid crystal network is effectively a rubber that stretches or relaxes on its own, without anyone applying a force," explains Lagerwall.
It turns out that the behavior of negative-order parameter liquid crystal rubbers is exactly opposite to that of normal liquid crystal rubbers. "Optically, when a normal liquid crystal rubber shows a certain color between crossed polarizers, the negative-order parameter version shows the complementary color," says Lagerwall. "Mechanically, when a normal liquid crystal rubber contracts along one direction and expands in the plane perpendicular to it, the negative-order parameter rubber expands along the first direction and shrinks in the perpendicular plane."
The researchers created their negative-order parameter liquid crystal rubbers in the form of millimeter-sized spherical shells, which they then cut into smaller pieces with various shapes. Depending on how the cut was made, a variety of shape-changing behaviors could be realized, showing that the system can function as a soft ‘actuator’, effectively an artificial muscle.
Because the negative- and positive-order liquid crystal rubbers act in opposite ways, they can be combined together to make a more effective composite actuator. When the positive-order actuator responds slowly, the negative-order one will respond quickly, and vice versa. From a fundamental physics point of view, the physical existence of this anti-ordered liquid crystal state, which was previously only theoretically predicted, opens the way for many interesting experiments, as well as the development of novel theories for the behavior of self-organizing soft matter.
This story is adapted from material from the University of Luxembourg, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.