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Researchers Gregor Pirnat, Matevž Marinčič, Miha Ravnik, and Matjaž Humar from the F5 department at the “J. Stefan” Institute and the Faculty of Mathematics and Physics at the University of Ljubljana have published a paper titled Quantifying Local Stiffness and Forces in Soft Biological Tissues Using Droplet Optical Microcavities in the journal PNAS. Researchers developed a method for quantitative measurements of mechanical properties of soft biological tissues and materials based on the measurements of »whispering gallery mode« spectra of optical resonances in droplet microresonators. They implanted the droplets in test samples (e.g. brain tissue) and determined the droplet’s deformation and shape with nanometer precision, by measuring small shifts of the optical resonances in spectra, measured at different positions in the droplet. The elastocapillary interaction couples the droplet to the deformation of the surrounding medium, enabling the determination of material properties of the medium. Because of the high sensitivity of optical resonances they could measure forces of only a few piconewtons at the droplet surface. The purpose of the method is elastography of soft biological tissues, ranging from mucus to muscle tissue.

The researchers Zala Korenjak and Asst. Prof. Matjaž Humar from the Laboratory for Bio-integrated Photonics, Department of Condensed Matter Physics at the Jožef Stefan Institute have published a paper in Physical Review X entitled Smectic and soap bubble optofluidic lasers. In the paper, they demonstrated for the first time that smectic and soap bubbles can be used as lasers. They doped the bubbles with a fluorescent dye and pumped them with an external laser to induce whispering gallery mode optical lasing. Bubbles made of smectic liquid crystals have a very thin and uniform wall and are extremely stable. Shifts in lasing wavelengths in the spectrum of the emitted light, which contained hundreds of regularly spaced sharp peaks, enabled the measurement of subtle size changes of just ten nanometers in a millimeter-sized bubble. This incredible precision allowed the bubbles to be used as one of the best pressure and electric field sensors developed to date. This unique physical system may in the future allow the study of novel optical and mechanical phenomena in thin films. Matjaž and Zala presented the research in video.

George Cordoyiannis from the Department of Condensed Matter Physics F5 and colleagues from KU Leuven (Belgium) have published an invited review article Review of high-resolution calorimetric studies of cyanoalkylbiphenyl and cyanoalkoxybiphenyl liquid crystals and related compounds in Liquid Crystals for the 50 years anniversary from the discovery of the important family of cyanobiphenyl compounds. This review gives an overview of different calorimetric methods and highlights important high-resolution calorimetry measurements on these compounds over the last 50 years.

Andrej Vilfan from the Department of Condensed Matter Physics F5 and colleagues from Germany and Great Britain have published an article in the journal Proceedings of the National Academy of Sciences with the title Nonreciprocal interactions give rise to fast ciliumsynchronization in finite systems. Motile cilia are hairlike organelles that, at sufficient density, can synchronize hydrodynamically with their neighbors to form a metachronal wave. We use a minimal model of a ciliary carpet that accounts for near-field hydrodynamic coupling between cilia and show that the interaction between cilia can be nonreciprocal. The collective dynamics of an array of cilia is therefore characterized by three different velocities and their directions: the direction of fluid transport, the direction of metachronal waves (phase velocity), and the direction of order propagation (group velocity). The latter determines the time scale of synchronization. Near-field nonreciprocal interactions can therefore give rise to rapid emergence of metachronal waves.

Andrej Vilfan from the Department of Condensed Matter Physics F5 and colleagues from Germany and Great Britain have published an article in the journal Physical Review Letters with the title Lorentz Reciprocal Theorem in Fluids with Odd Viscosity. Fluids with odd viscosity break the time reversal symmetry and consequently the Lorentz reciprocal theorem does not hold. Here we report a way of restoring its validity and demostrate how it can be applied to predict the motion of active particles in such a fluid.