Force dipoles and stable local defects on fluid vesicles

Abstract

An exact description is provided of an almost spherical fluid vesicle with a fixed area and a fixed enclosed volume locally deformed by external normal forces bringing two nearby points on the surface together symmetrically. The conformal invariance of the two-dimensional bending energy is used to identify the distribution of energy as well as the stress established in the vesicle. While these states are local minima of the energy, this energy is degenerate; there is a zero mode in the energy fluctuation spectrum, associated with area and volume preserving conformal transformations, which breaks the symmetry between the two points. The volume constraint fixes the distance $S$, measured along the surface, between the two points; if it is relaxed, a second zero mode appears, reflecting the independence of the energy on $S$; in the absence of this constraint a pathway opens for the membrane to slip out of the defect. Logarithmic curvature singularities in the surface geometry at the points of contact signal the presence of external forces. The magnitude of these forces varies inversely with $S$ and so diverges as the points merge; the corresponding torques vanish in these defects. The geometry behaves near each of the singularities as a biharmonic monopole, in the region between them as a surface of constant mean curvature, and in distant regions as a biharmonic quadrupole. Comparison of the distribution of stress with the quadratic approximation in the height functions points to shortcomings of the latter representation. Radial tension is accompanied by lateral compression, both near the singularities and far away, with a crossover from tension to compression occurring in the region between them.

26 pages, 10 figures

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• Geometry of surface mediated interactions
• Descriptions of membrane mechanics from microscopic and effective two-dimensional perspectives
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Cited by in corpus (6)

• Fluid Vesicles in Flow
• Toroidal membrane vesicles in spherical confinement
• Some observations on variational elasticity and its application to plates and membranes
• Equatorial deformation of homogeneous spherical fluid vesicles by a rigid ring
• Shear stresses in fluid and solid membranes with bending elasticity
• Small equatorial deformation of homogeneous spherical fluid vesicles