  Ultrasound is increasingly being used to modulate the properties of
biological membranes for applications in drug delivery and neuromodulation.
While various studies have investigated the mechanical aspect of the
interaction such as acoustic absorption and membrane deformation, it is not
clear how these effects transduce into biological functions, for example,
changes in the permeability or the enzymatic activity of the membrane. A
critical aspect of the activity of an enzyme is the thermal fluctuations of its
solvation or hydration shell. Thermal fluctuations are also known to be
directly related to membrane permeability. Here solvation shell changes of
lipid membranes subject to an acoustic impulse were investigated using a
fluorescence probe, Laurdan. Laurdan was embedded in multi-lamellar lipid
vesicles in water, which were exposed to broadband pressure impulses of the
order of 1MPa peak amplitude and 10{\mu}s pulse duration. An instrument was
developed to monitor changes in the emission spectrum of the dye at two
wavelengths with sub-microsecond temporal resolution. The experiments show that
changes in the emission spectrum, and hence the fluctuations of the solvation
shell, are related to the changes in the thermodynamic state of the membrane
and correlated with the compression and rarefaction of the incident sound wave.
The results suggest that acoustic fields affect the state of a lipid membrane
and therefore can potentially modulate the kinetics of channels and proteins
embedded in the membrane.
