AI News, Liquid crystal shells: 'Smart' material enables novel applications in autonomous driving and robotics

Liquid crystal shells: 'Smart' material enables novel applications in autonomous driving and robotics

Liquid Crystals, already widely used in flat-screen TVs, are materials that are in a state between solid and liquid.

Liquid Crystal shells, only fractions of a millimeter in size so they can easily be applied to surfaces, have several unique properties that could be utilized in engineering: As they reflect light highly selectively, they can be arranged into patterns that are readable for machines, akin to a QR code, adding coded information to objects.

As the Liquid Crystal shells reflect light 'omnidirectionally' meaning that beholders see the same pattern regardless of their position and viewing angle, the patterns can even be read by moving objects.

Together with computers to interpret these changes, the shells could be used as sensors, for example as pressure sensors in the fingertips of robots enabling tactile feeling which is currently hard to achieve in robotic engineering.

'Our hope is that the article can stimulate future research on liquid crystalline materials into new directions that are in line with the current societal developments,' he said.

Liquid Crystals Shells Have Potential for Autonomous Vehicles, Robotics and Security Technologies

A team from the University of Luxembourg has discovered the potential of liquid crystal shells in a number of new applications, including autonomous vehicles, anti-counterfeiting technology and a new class of sensors.

The shells can highly selectively reflect light and can be arranged into patterns that are readable for machines like a QR code to add coded information to objects.

“This could become important especially in indoors applications where GPS devices don't work.” According to the study, in order to prevent the shells from collapsing, the researchers added surface‐active stabilizers to the isotropic phases.

The liquid crystal shells reflect light omnidirectionally—where the same pattern is seen regardless of viewing angle and can even be read by moving objects.

Liquid crystal shells could also be used in fire exit signage on walls inside buildings that only become visible when the temperature exceeds a certain threshold.

High-fidelity spherical cholesteric liquid crystal Bragg reflectors generating unclonable patterns for secure authentication

Figure 1 shows the optical appearance of well-aligned cholesteric spheres with radii about 100 μm, each surrounded by identical spheres arranged more or less in a close-packed hexagonal pattern, for the cases of droplets (left column) and shells of thickness decreasing stepwise from left to right, from 40 μm to 5 μm.

In an improved approach, we transferred shells prepared with an inner water-glycerol solution of polyvinyl alcohol (PVA) into a new outer aqueous phase with substantially lower PVA and glycerol content, giving rise to an osmotic flow of water into the internal aqueous droplet.

Another very important advantage of using shells rather than droplets is that the achieved uniform alignment is more robust, thanks to the tight confinement of a small volume of cholesteric between two closely spaced tangential-aligning interfaces.

However, even using just a single achiral reactive mesogen we can polymerise a sufficient fraction of the shell to ensure robustness against mechanical shock and shells pushing against each other, thereby allowing easy manipulation and greatly expanding the application opportunities for the shells.

We used the commonly employed commercially available reactive mesogen RM-257, added at a fraction between 5 and 20 wt.-% to our basic cholesteric mixture, as well as a photoinitiator to start the polymerisation by UV light irradiation.

In all cases, the optical quality of the shells after polymerisation is excellent, with very well defined optical communication and intense main reflection from the shell centre and no apparent defect generation seen either in transmission or reflection.

Although the shells are distorted so strongly that the central reflection spot increases almost 20-fold in area, the cross communication pattern is fully retained immediately after the pressure is removed, as shown in pane (d).

Importantly, these shells carry no protective coating, in contrast to the system studied by Lee et al.11, simplifying sample production and also enhancing the optical properties, since any additional layer will introduce reflection and scattering.

The three different pitch lengths investigated were such that their normal incidence reflections, seen in the central spot in each shell, are centred at the wavelengths 0.96 μm (infrared, IR), 0.67 μm (red, R) and 0.57 μm (green, G), respectively.

The microscope used for illuminating and imaging the sample integrates light over a certain range of propagation directions, within cones with opening angles γi and γr for illuminating and reflected light, respectively.

5, cases (d) and (e) are within the bidirectional communication range, as the incoming and outgoing beams, within the sample, have angles α and β with respect to the vertical direction that are clearly less than γr′.

The numerical aperture value of the objective used is specified for the imaging light cone, whereas the value γi′ of the illumination light cone can be regulated with an aperture diaphragm, being effectively even larger when the aperture is fully open.

The possibility to randomly combine shells with different photonic band gaps in a single token, with bi- as well as uni-directional communication at intermediate wavelengths, thus vastly extends the possibilities of creating unique and highly distinct patterns using these shells, with great potential in the field of secure authentication.

It might also be feasible, at least in principle, to arrange them in the same order as in an original token of the most rudimentary design, where the shells are regularly ordered in an equilibrium arrangement, given sufficient time and access to extremely precise micromanipulators.

The unclonability here derives from how the shells are allowed to arrange themselves at production within the hosting medium, with random combinations of shells with different values of the cholesteric pitch length, preferrably arrested in a non-equilibrium configuration, as just discussed, or possibly in a randomly packed arrangement29.

Unclonability can also be discussed in a non-physical sense, referring to the question of whether or not the optical pattern emerging from a specific array of shells could be reproduced via computer simulation by an adversary with limited computing resources that got access to this array.

The optical cross-communication demonstrated in this paper suggests non-local dependences in the definition of the pattern but a quantification of the degree of non-locality, thus describing how hard it is to simulate with a computer the challenge-response behavior of an array of shells, is a question that we will address in our future investigations.

The ideal degree of mechanical sturdiness should be just enough to allow the shells to be processed after production, as required in order to realise the intended token with non-identical shells made from different cholesteric mixtures.

They clearly withstand the conditions of normal usage, but the abnormal deformation during an attempt to physically modify a token would lead to irreversible damage due to the partially liquid state of the shells.

Combining the ease in generating PUFs from cholesteric liquid crystal shells, that respond with dynamic, unpredictable, unique and unclonable photonic patterns to different light stimuli, with the tamper-resistance ensured by a tailored degree of polymerisation, we conclude that this new configuration of cholesteric liquid crystals constitutes an extremely interesting component for realising future high-security authentication and identification protocols.

Our experiments demonstrate that the optical quality of shells is greatly enhanced compared to that of droplets, in particular after a stage of osmotic thinning of the shell, which effectively removes oily streak defects.

Adding the enormous variation possibilities given by combining shells with non-identical photonic band gaps, where each combination opens a specific cross communication channel, it is clear that cholesteric micro shells have outstanding potential in security, thereby aiding to solve a problem of great current societal importance.

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