Measurement of water vapor diffusion in nanoscale polymer films by frequency-domain probe beam deflection

Microscale, bendable thermoreflectance sensor for local measurements of the thermal effusivity of biological fluids and tissues

Measurements of the thermal transport properties of biological fluids and tissues are important for biomedical applications such as thermal diagnostics and thermal therapeutics. Here, we describe a microscale thermoreflectance sensor to measure the thermal effusivity of fluids and biological samples in a minimally invasive manner. The sensor is based on ultrafast optical pump-probe techniques and employs a metal-coated optical fiber as both a photonic waveguide and a local probe. Calibration of the sensor with five liquids shows that the percentage deviation between experimentally measured effusivity and literature values is on average <3%. We further demonstrate the capability of the sensor by measuring the thermal effusivity of vegetable oil, butter, pork liver, and quail egg white and yolk. We relate the thermal effusivity of the samples to their composition and water content, and establish our technique as a powerful and flexible method for studying the local thermal transport properties of biological materials.

Light-triggered thermal conductivity switching in azobenzene polymers

Materials that can be switched between low and high thermal conductivity states would advance the control and conversion of thermal energy. Employing in situ time-domain thermoreflectance (TDTR) and in situ synchrotron X-ray scattering, we report a reversible, light-responsive azobenzene polymer that switches between high (0.35 W m-1 K-1) and low thermal conductivity (0.10 W m-1 K-1) states. This threefold change in the thermal conductivity is achieved by modulation of chain alignment resulted from the conformational transition between planar (trans) and nonplanar (cis) azobenzene groups under UV and green light illumination. This conformational transition leads to changes in the π-π stacking geometry and drives the crystal-to-liquid transition, which is fully reversible and occurs on a time scale of tens of seconds at room temperature. This result demonstrates an effective control of the thermophysical properties of polymers by modulating interchain π-π networks by light.