Single Microbubble Measurements of Lipid Monolayer Viscoelastic Properties for Small-Amplitude Oscillations

Abstract

Lipid monolayer rheology plays an important role in a variety of interfacial phenomena, the physics of biological membranes, and the dynamic response of acoustic bubbles and drops. We show here measurements of lipid monolayer elasticity and viscosity for very small strains at megahertz frequency. Individual plasmonic microbubbles of 2-6 μm radius were photothermally activated with a short laser pulse, and the subsequent nanometer-scale radial oscillations during ring-down were monitored by optical scatter. This method provided average dynamic response measurements of single microbubbles. Each microbubble was modeled as an underdamped, linear oscillator to determine the damping ratio and eigenfrequency, and thus the lipid monolayer viscosity and elasticity. Our non-isothermal measurement technique revealed viscoelastic trends for different lipid shell compositions. We observed a significant increase in surface elasticity with lipid acyl chain length for 16 to 20 carbons, and this effect was explained by an intermolecular forces model that accounts for lipid composition, packing and hydration. The surface viscosity was found to be equivalent for these lipid shells. We also observed an anomalous decrease in elasticity and increase in viscosity when increasing the acyl chain length from 20 to 22 carbons. These results illustrate the use of a novel nondestructive optical technique to investigate lipid monolayer rheology in new regimes of frequency and strain, possibly elucidating phase behavior, as well as how the dynamic response of a microbubble can be tuned by the lipid intermolecular forces.

Phospholipid Oxygen Microbubbles for Image-Guided Therapy

“…This result is consistent with contrast agent studies that compared differing acyl-chain phospholipids for increase contrast persistence and circulation in vitro and in vivo [7]. Longer acyl chains increase intermolecular forces between the phospholipids [21], thereby increasing shell rigidity [22,23], decreasing shell permeability [24,25], and increasing microbubble lifetime [7,26].…”

High-precision acoustic measurements of the nonlinear dilatational elasticity of phospholipid coated monodisperse microbubbles

“…19,37,56,57 For DSPC based bubbles the lower limit of χ is similar to that of DPPC based bubbles however, even higher maximum values have been reported, up to 3.5 N/m. 20,38,58 To date, the nature of the reported differences remains unclear. The length of the PEG chain, the length of the primary lipid molecules, temperature, microbubble production method, resting time after synthesis, and phase behavior and microstructure formation of the lipids in the coating may all play a role.…”

Bubble puzzles: From fundamentals to applications

“…[36]. (c) Eigen frequency f 0 of a sound-driven coated with DPPC monolayers as function of the ambient bubble radius (blue data points, from [37]). The solid blue line shows the fit to the Marmottant model [38], whereas the dashed line shows the Minnaert frequency (3).…”