Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

New method inverts the self-assembly of liquid crystals

15.04.2019

In liquid crystals, molecules automatically arrange themselves in an ordered fashion. Researchers from the University of Luxembourg have discovered a method that allows an anti-ordered state, which will enable novel material properties and potentially new technical applications, such as artificial muscles for soft robotics. They published their findings in the scientific journal Science Advances.

The research team of Prof. Jan Lagerwall at the University of Luxembourg studies the characteristics of liquid crystals, which can be found in many areas ranging from cell membranes in the body to displays in many electronic devices. The material combines liquid-like mobility and flexibility and long-range order of its molecules; the latter is otherwise a typical feature of solid crystals.


The image shows the actuation of a cup-shaped object (half sphere) slowly folding into an ellipsoid upon heating and return back to cup-shape while cooling. This object too shows the minimizing its surface area upon heating and get back to the original state upon cooling.

Credit: University of Luxembourg

Usage Restrictions: The image may only be used with appropriate caption or credit.

This gives rise to remarkable properties that render liquid crystals so versatile that they are chosen for carrying out vital functions by nature and by billion-dollar companies alike.

Many of a material's properties depend on the way its molecules are arranged. Since the late 1930s, physicists use a mathematical model to describe the molecular order of liquid crystals. The so-called order parameter assigns a number that indicates 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 that describes an "anti-ordered" state, where the molecules would avoid a certain direction rather than align along it.

So far, this negative range remained strictly hypothetical, as no liquid crystal developed 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.

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," explains Jan Lagerwall.

This is exactly what V.S.R. Jampani, the main author of the paper, and co-workers achieved 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 Dr. 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 of the 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, the 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" says Prof. Lagerwall.

It turns out that the behavior of the negative order parameter liquid crystal rubber 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. 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," Lagerwall explains.

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 behavior 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, this opens for interesting ways to combine the two, to make a more effective composite actuator, for instance for soft robotics. When the positive-order actuator responds slowly, the negative-order one actuates quickly, and vice versa.

From a fundamental physics point of view, the physical existence of the previously only theoretically predicted anti-ordered liquid crystal state opens for many interesting experiments as well as theory development for the behavior of self-organizing soft matter.

Thomas Klein | EurekAlert!
Further information:
http://dx.doi.org/10.1126/sciadv.aaw2476

Further reports about: artificial muscle fundamental physics liquid crystal rubber

More articles from Materials Sciences:

nachricht Freiburg researcher investigate the origins of surface texture
17.02.2020 | Albert-Ludwigs-Universität Freiburg im Breisgau

nachricht Understanding Metal Ion Release from Hip Implants
17.02.2020 | Max-Planck-Institut für Eisenforschung GmbH

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: A step towards controlling spin-dependent petahertz electronics by material defects

The operational speed of semiconductors in various electronic and optoelectronic devices is limited to several gigahertz (a billion oscillations per second). This constrains the upper limit of the operational speed of computing. Now researchers from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, and the Indian Institute of Technology in Bombay have explained how these processes can be sped up through the use of light waves and defected solid materials.

Light waves perform several hundred trillion oscillations per second. Hence, it is natural to envision employing light oscillations to drive the electronic...

Im Focus: Freiburg researcher investigate the origins of surface texture

Most natural and artificial surfaces are rough: metals and even glasses that appear smooth to the naked eye can look like jagged mountain ranges under the microscope. There is currently no uniform theory about the origin of this roughness despite it being observed on all scales, from the atomic to the tectonic. Scientists suspect that the rough surface is formed by irreversible plastic deformation that occurs in many processes of mechanical machining of components such as milling.

Prof. Dr. Lars Pastewka from the Simulation group at the Department of Microsystems Engineering at the University of Freiburg and his team have simulated such...

Im Focus: Skyrmions like it hot: Spin structures are controllable even at high temperatures

Investigation of the temperature dependence of the skyrmion Hall effect reveals further insights into possible new data storage devices

The joint research project of Johannes Gutenberg University Mainz (JGU) and the Massachusetts Institute of Technology (MIT) that had previously demonstrated...

Im Focus: Making the internet more energy efficient through systemic optimization

Researchers at Chalmers University of Technology, Sweden, recently completed a 5-year research project looking at how to make fibre optic communications systems more energy efficient. Among their proposals are smart, error-correcting data chip circuits, which they refined to be 10 times less energy consumptive. The project has yielded several scientific articles, in publications including Nature Communications.

Streaming films and music, scrolling through social media, and using cloud-based storage services are everyday activities now.

Im Focus: New synthesis methods enhance 3D chemical space for drug discovery

After helping develop a new approach for organic synthesis -- carbon-hydrogen functionalization -- scientists at Emory University are now showing how this approach may apply to drug discovery. Nature Catalysis published their most recent work -- a streamlined process for making a three-dimensional scaffold of keen interest to the pharmaceutical industry.

"Our tools open up whole new chemical space for potential drug targets," says Huw Davies, Emory professor of organic chemistry and senior author of the paper.

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

70th Lindau Nobel Laureate Meeting: Around 70 Laureates set to meet with young scientists from approx. 100 countries

12.02.2020 | Event News

11th Advanced Battery Power Conference, March 24-25, 2020 in Münster/Germany

16.01.2020 | Event News

Laser Colloquium Hydrogen LKH2: fast and reliable fuel cell manufacturing

15.01.2020 | Event News

 
Latest News

Sweet beaks: What Galapagos finches and marine bacteria have in common

20.02.2020 | Life Sciences

Social networks reveal dating in blue tits

20.02.2020 | Life Sciences

More focus and comfort at telephone workstations

20.02.2020 | Communications Media

VideoLinks
Science & Research
Overview of more VideoLinks >>>