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John O | June 2017

Researchers develop ways to control molecular vibrations that transmit heat


a team of researchers at the u.s. department of energy’s (doe) brookhaven national laboratory (upton, n.y.) have used the newly constructed inelastic x-ray scattering (ixs) beamline at the national synchrotron light source ii (nsls-ii) to track the dynamic molecular features of soft materials, including the vibrational waves that transmit energy.

 



brookhaven lab members of the research team at the ixs beamline of the national
synchrotron light source ii. (brookhaven national laboratory)

 

by revealing these molecular dynamics in materials such as polymers or liquid crystal compounds, the researchers believe that they can be controlled and lead to thermal and acoustic insulators, ways to convert waster heat into electricity or light into mechanical motion, according to a report from the lab.

 

the article explained, “their findings show that the nanoscale structural changes that occur with increasing temperature—as the liquid crystals become less ordered—dramatically disrupt the flow of vibrational waves. thus, choosing or changing the ‘phase,’ or arrangement of molecules, could control the vibrations and the flow of energy.”

 

researchers believe this process could be used on other materials like biological membranes or complex fluids. potentially, this could allow scientists to observe details like gas exchange in lungs or how molecules interact in cell membranes.

 

the construction of the nsls-ii allowed this discovery to be made. the article noted, “at the ixs beamline, scientists bombard samples with these x-rays and measure the energy they give up or gain with a precision to within two thousandths of an electron volt, as well as the angle at which they scatter off the sample—even at very small angles.”

 

scientists were able to precisely record how much energy was needed to make some molecules vibrate in a wave-like motion and the scattering angle in different directions and over different length scales.

 

the article continued, “in the liquid crystal study, the brookhaven lab scientists and their collaborators at kent state university and the university at albany made measurements at three different temperatures as the material went from an ordered, crystalline phase through transitions to a less-ordered ‘smectic’ state, and finally an ‘isotropic’ liquid. they easily detected the propagation of vibrational waves through the most ordered phase, and showed that the emergence of disorder ‘killed’ the propagation of low energy ‘acoustic shear’ vibrations. acoustic shear vibrations are associated with a compression of the molecules in a direction perpendicular to the direction of propagation.”

 

the researchers also witnessed vibrations associated with molecular tilt, which can interact and absorb light. this property can be changed not only by temperature but also by the application of external electric or magnetic fields, which could lead to phononic or optomechanical materials.

 

studies of other soft materials such as block copolymers, nanoparticle assemblies, lipid membranes, and other liquid crystals will continue on the next few months.

 

the research was recently published in nano letters. the abstract stated:

 

“the investigation of phononic collective excitations in soft matter systems at the molecular scale has always been challenging due to limitations of experimental techniques in resolving low-energy modes. recent advances in inelastic x-ray scattering (ixs) enabled the study of such systems with unprecedented spectral contrast at mev excitation energies.

 

“in particular, it has become possible to shed light on the low-energy collective motions in materials whose morphology and phase behavior can easily be manipulated, such as mesogenic systems. the understanding of collective mode behavior with a q-dependence is the key to implement heat management based on the control of a sample structure.

 

“the latter has great potential for a large number of energy-inspired innovations. as a first step toward this goal, we carried out high contrast ixs measurements on a liquid crystal sample, d7aob, which exhibits solid-like dynamic features, such as the coexistence of longitudinal and transverse phononic modes.

 

“for the first time, we found that these terahertz phononic excitations persist in the crystal, smectic a, and isotropic phases. furthermore, the intermediate smectic a phase is shown to support a van der waals-mediated nonhydrodynamic mode with an optical-like phononic behavior.

 

“the tunability of the collective excitations at nanometer–terahertz scales via selection of the sample mesogenic phase represents a new opportunity to manipulate optomechanical properties of soft metamaterials.”

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