The Phonon Theory of Liquids and Biological Fluids: Developments and Applications

Among the three basic states of matter (solid, liquid, and gas), the liquid state has always eluded general theoretical approaches for describing liquid energy and heat capacity. In this Viewpoint, we derive the phonon theory of liquids and biological fluids stemming from Frenkel's microscopic picture of the liquid state. Specifically, the theory predicts the existence of phonon gaps in vibrational spectra of liquids and a thermodynamic boundary in the supercritical state. Direct experimental evidence reaffirming these theoretical predictions was achieved through a combination of techniques using static compression X-ray diffraction and inelastic X-ray scattering on deeply supercritical argon in a diamond anvil cell. Furthermore, these findings inspired and then led to the discovery of phonon gaps in liquid crystals (mesogens), block copolymers, and biological membranes. Importantly, phonon gaps define viscoelastic crossovers in cellular membranes responsible for lipid self-diffusion, lateral molecular-level stress propagation, and passive transmembrane transport of small molecules and solutes. Finally, molecular interactions mediated by external stimuli result in synaptic activity controlling biological membranes' plasticity resulting in learning and memory. Therefore, we also discuss learning and memory effects-equally important for neuroscience as well as for the development of neuromorphic devices-facilitated in biological membranes by external stimuli.

Automated summary

This viewpoint discusses the phonon theory of liquids and biological fluids. The theory predicts the existence of phonon gaps in vibrational spectra of liquids and a thermodynamic boundary in the supercritical state. Experimental evidence reaffirms these predictions and also leads to the discovery of phonon gaps in liquid crystals, block copolymers, and biological membranes. Phonon gaps define viscoelastic crossovers in cellular membranes and play a role in lipid self-diffusion, stress propagation, and transmembrane transport. External stimuli-mediated molecular interactions control the plasticity of biological membranes, resulting in learning and memory.