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We demonstrate that time-domain Brillouin scattering (TDBS), a technique based on an ultrafast pump-probe approach, is sensitive to phase transitions and apply it to the study of structural changes in 8CB liquid crystals at different temperatures across the isotropic, nematic, smectic, and crystalline phases. We investigate the viscoelastic properties of 8CB squeezed in a narrow gap, from the nanometer to submicrometer thickness range, and conclude on the long-range molecular structuring of the smectic phase. These TDBS results reveal that confinement effects favor structuring of the smectic phase into a crystallinelike phase that can be observed at wide distances far beyond the molecular dimensions. [ABSTRACT FROM AUTHOR]

A number of nascent coating and micromanufacturing processes involve particles sprayed at a high velocity, which impact a substrate, deform, and adhere. Successful bonding between the particles and the substrate requires impact velocities higher than the so-called critical adhesion velocity. This critical velocity is influenced by the temperature of the substrate, a variable that we isolate in this work by conducting single-particle impacts on a variable-temperature substrate for three systems (Al–Al, Sn–Sn, and Ti–Ti) with particles individually selected within a narrow size distribution. Our results quantitatively connect the increase in substrate temperature to a significant lowering of the critical velocity, which we attribute to the lower dynamic strength of the thermally softened substrate. The data are generally consistent with expectations for bonding being controlled by a hydrodynamic process of jetting upon impact. [ABSTRACT FROM AUTHOR]

We present an optical technique based on ultrafast photoacoustics to determine the local temperature distribution profile in liquid samples in contact with a laser heated optical transducer. This ultrafast pump-probe experiment uses time-domain Brillouin scattering (TDBS) to locally determine the light scattering frequency shift. As the temperature influences the Brillouin scattering frequency, the TDBS signal probes the local laser-induced temperature distribution in the liquid. We demonstrate the relevance and the sensitivity of this technique for the measurement of the absolute laser-induced temperature gradient of a glass forming liquid prototype, glycerol, at different laser pump powers— i.e., different steady state background temperatures. Complementarily, our experiments illustrate how this TDBS technique can be applied to measure thermal diffusion in complex multilayer systems in contact with a surrounding liquid. [ABSTRACT FROM AUTHOR]

[Display omitted] • LIESST switches spin-crossover systems from low to high spin states. • Photoinduced LS to HS up conversion and HS to LS relaxation compete. • High light irradiance shifts photo-stationary equilibrium towards high temperature. • Raman and magnetic data evidence the phenomenon. • A simple model explains photo-stationary stabilization towards higher temperature. The Light-Induced Excited Spin-State Trapping (LIESST) process allows tuning the electronic state of spin-crossover materials from low (LS) to high spin (HS) states. The photoinduced HS state is long-lived, up to a characteristic temperature, T(LIESST), above which the system relaxes rapidly to the LS state. We study the effect of light irradiance on competing LS → HS up-conversion and HS → LS thermal relaxation on the [Fe(phen) 2 (NCS) 2 ] system. Raman spectroscopy and magnetic measurements are used to investigate this phenomenon. An empiric model describes the competition between up conversion and thermal relaxation mechanisms and the stabilization of photo-stationary state towards higher temperature. [ABSTRACT FROM AUTHOR]

The Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material was designed to exhibit a 75 K wide thermal hysteresis around room temperature, associated with a bistability between the low temperature MnIIIFeII tetragonal phase and the high temperature MnIIFeIII cubic phase. We have recently shown that a single laser pulse can be used to drive, at room temperature, the photoinduced phase transition from the MnIIIFeII tetragonal phase to the MnIIFeIII cubic phase. By using a specific powder sample streaming technique, we could monitor by time‐resolved X‐ray diffraction how the ultrafast out‐of‐equilibrium dynamics, imposed by the structural relaxation around the local photoinduced charge transfer, changes with the fluence of the laser pulse. In this paper we present a detailed structural analysis of the crystalline reorganizations associated with the charge‐transfer in different regimes of photoexcitation. In the low excitation polaronic regime, the photoinduced charge transfer is local and accompanied by an anisotropic distortion of the Mn coordination sphere and of the tetragonal lattice. In the high excitation regime, a macroscopic and cooperative charge‐transfer phase transition occurs towards the cubic lattice, where the coordination around the Mn reaches octahedral symmetry. [ABSTRACT FROM AUTHOR]