A new method reverses self-assembly of liquid crystals

A new method reverses self-assembly of liquid crystals

Operation of a cup-shaped object (half-sphere) slowly folding into an ellipsoid during heating and returning to the form of a cut while cooling. This object also shows the possibility of minimizing its surface during heating and returning to its initial state during cooling. Credit: University of Luxembourg

In liquid crystals, molecules organize automatically in an orderly fashion. Researchers at the University of Luxembourg have discovered a method for an anti-ordered state, which will allow new properties of materials and potentially new technical applications, such as artificial muscles for soft robotics. They published their results in the scientific journal Progress of science.

The research team of Professor Jan Lagerwall of the University of Luxembourg is studying the characteristics of liquid crystals, present in many fields, ranging from the cell membranes of the body to the screens of many electronic devices. The material combines fluid mobility and flexibility with a long-term order of its molecules; the latter is also a characteristic characteristic of solid crystals. This gives remarkable properties that make liquid crystals so versatile that they are chosen to perform vital functions by nature and by companies worth billions of dollars.

Many of the properties of a material depend on how its molecules are arranged. Since the late 1930s, physicists have used a mathematical model to describe the molecular order of liquid crystals. The parameter "order" assigns a number indicating the degree of order of the molecules. This model uses a positive range to describe the liquid crystals we are used to. It can also assign a negative range describing an "anti-ordered" state, in which the molecules would avoid a certain direction rather than aligning along it.

Up to now, this negative interval has remained strictly hypothetical, with no liquid crystal developing an anti-ordinate state in practice. Standard theories for liquid crystals suggest that such a state is possible, but would not be stable. "You can compare that to a slide that has a very slight bump in the middle.You can slow down when you hit the bump, in our case the unstable unsteady state, but not enough to stop you, and so you will go Up to a stable level, the minimum of overall energy, where you inevitably end up with a positive order.If you could stop running, a negative range would be possible, "says Jan Lagerwall.

That's exactly what V.S.R. Jampani, the main author of the article, and his collaborators have made for the first time their study. "To prevent the system from reaching the minimum of overall energy, it must be gently cured into a poorly connected network, while dissolving it in a normal liquid solvent," says Dr. Jampani. "This grating is then stretched in all directions of a plane or compressed in a single direction perpendicular to the plane, so that the molecules constituting the grating align in the plane, but without any particular direction in this plane . " When the solvent evaporates, the liquid crystal phase is formed and, due to the particular stretching of the grating in the plane, it is forced to adopt the parameter state of ## EQU1 ## Negative order in which the molecules avoid the direction of the normal to the plane. "This liquid crystal has no choice but to install with the minimum of secondary energy, because the minimum of overall energy is made inaccessible by the network," adds Lagerwall.

When the network is reinforced by a second polymerization cycle, the behavior as a function of temperature can be studied. "Liquid crystal networks are fascinating for positive and negative order parameters, because the order, or the anti-order, in combination with the polymer network allows it to spontaneously change shape. In response to changes in temperature, the liquid crystal network is a rubber that stretches or relaxes alone, without anyone forcibly applying it, "says Professor Lagerwall.

It turns out that the behavior of the negative order parameter liquid crystal rubber is exactly opposite to that of the normal liquid crystal rubbers. "Optically, when a normal liquid crystal rubber has some color between crossed polarizers, the negative order parameter version indicates the complementary color." Mechanically, when a normal liquid crystal rubber contracts in one direction and expands in the plane perpendicular to it, the negative The rubber control parameter expands in the first direction and contracts in the perpendicular plane, "explains Lagerwall.

The researchers created their negative-order liquid crystal rubbers in the form of millimeter-sized spherical hulls, which they then cut into smaller pieces of various shapes. Depending on how the cut was made, a variety of shape-changing behaviors can be performed, showing that the system can function as a flexible "actuator," effectively an artificial muscle. Since liquid crystal rubbers, of negative and positive order, act in opposite ways, it is interesting to combine the two to create a more efficient composite actuator, for example for soft robotics. When the positive order actuator responds slowly, the negative order acts quickly, and vice versa. From the point of view of fundamental physics, the physical existence of the anti-ordered liquid crystalline state, theoretically predicted so far, opens the way to many interesting experiments as well as the development of the theory of behavior of the self-organized soft material.

Can a fluid-like liquid-like material retain its structural order like crystals?

More information:
"Liquid crystal elastomer shell actuators with negative order parameter" Progress of science (2019). DOI: 10.1126 / sciadv.aaw2476, https://advances.sciencemag.org/content/5/4/eaaw2476

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A new method reverses the self-assembly of liquid crystals (April 12, 2019)
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