The Role of Optical Projection on Vesicle Fluctuations

The optical spectroscopy of thermally induced shape fluctuations of giant unilamellar vesicles (GUVs) has been widely used as a method to extract mechanical information about fluid membranes [1]. Working with the model system of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipids, we re-examine this methodology and discuss how the projection of fluctuations within the focal depth of the microscope may affect the inferred value of the bending modulus (and the surface tension). Within a Gaussian approximation we derive an analytical expression for a mode spectrum that varies with the ratio of the focal depth to the vesicle size. A comparison of our model with the existing approach [2-4] (that compares experiments with the equatorial fluctuations, without averaging over the focal depth) shows a significant and systematic decrease in the inferred value of the bending modulus. The new procedure is found to be in good agreement with the values measured through X-ray scattering and other micromechanical manipulation techniques [5].

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[4] P. Méléard et al., Eur. Phys. J. E 34, 116 (2011)
[5] J. F. Nagle, Farad. Discuss. 161, 11 (2013)