The colorful scattering pattern at left reveals nanoscale structural information about the layered smectic phase of a liquid crystal compound; The graph (top, right) represents inelastic x-ray scattering measurements from the smectic phase. The out-of-phase rocking back-and-forth of these molecules matches the frequency of infrared light.
The colorful scattering pattern at left reveals nanoscale structural information about the layered smectic phase of a liquid crystal compound; The graph (top, right) represents inelastic x-ray scattering measurements from the smectic phase. The out-of-phase rocking back-and-forth of these molecules matches the frequency of infrared light.

Scientists in the US have produced an innovative approach to tracking and controlling dynamic molecular vibrations that transmit waves of heat, sound, and other forms of energy. Manipulating these vibrational waves in soft materials – such as polymers and liquid crystal compounds – could help their wider use and the development of energy-inspired applications, including thermal and acoustic insulators, and methods for converting waste heat into electricity or light into mechanical motion.

The team, whose work was published in Nano Letters [Bolmatov et al. Nano Lett. (2017) DOI: 10.1021/acs.nanolett.7b01324], used a new inelastic x-ray scattering (IXS) beamline to assess the propagation of vibrations in a liquid crystal compound over three different phases. Nanoscale structural changes that occur with increasing temperature, while the liquid crystals become less ordered, were found to affect significantly the flow of vibrational waves. This means that choosing or changing the “phase” – the arrangement of molecules – allows the dynamic properties of the material to be altered, and the vibrations and flow of energy to be brought under control.

“the technical properties of this beamline enable us to precisely locate the vibrations and track their propagation in different directions over different length scales – even in materials that lack a well-ordered solid structure”Dima Bolmatov

Samples were bombarded by the x-rays to measure the energy they release or gain very precisely, as well as the angle at which they scatter off the sample. This informs on how much energy it takes for some molecules to vibrate in a wave-like motion, while the scattering angle probes the vibrations propagating over different length scales inside the sample. As lead author Dima Bolmatov said, “the technical properties of this beamline enable us to precisely locate the vibrations and track their propagation in different directions over different length scales – even in materials that lack a well-ordered solid structure”.

Measurements were made at three different temperatures as the material changed through its ordered, crystalline phase through transitions to a less-ordered “smectic” state, and finally an “isotropic” liquid. The propagation of vibrational waves through the most ordered phase was demonstrated, as well as the emergence of disorder that “killed” the spread of low energy “acoustic shear” vibrations, which are linked to a compression of the molecules in a direction perpendicular to the direction of propagation.

The study advances the potential for new phononic or optomechanical applications in which sound or light combines with the mechanical vibrations, so that control of the material based on the application of external light and sound can be achieve. The team will continue their research on the dynamic properties of soft matter materials, especially as there are many with interesting molecular structures and unexplored nanoscale behaviour to assess, such as block copolymers, nanoparticle assemblies, lipid membranes, and other liquid crystals.