Eur. Phys. J. E (2010) 31: 333-342
https://doi.org/10.1140/epje/i2010-10579-1

Regular Article

On the surface tension of fluctuating quasi-spherical vesicles

¹ Laboratoire Matière et Systèmes Complexes (MSC), UMR 7057 CNRS and Université Paris Diderot-Paris 7, Bât. Condorcet, CC 7056, 75205, Paris, France
² Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, Building 1520, DK-8000, Aarhus C, Denmark

^* e-mail: camilla.barbetta@m4x.org
^** e-mail: jean-baptiste.fournier@univ-paris-diderot.fr

Received: 7 November 2009
Accepted: 22 February 2010
Published online: 19 March 2010

Abstract

We calculate the stress tensor for a quasi-spherical vesicle and we thermally average it in order to obtain the actual, mechanical, surface tension of the vesicle. Both closed and poked vesicles are considered. We recover our results for by differentiating the free energy with respect to the proper projected area. We show that may become negative well before the transition to oblate shapes and that it may reach quite large negative values in the case of small vesicles. This implies that spherical vesicles may have an inner pressure lower than the outer one.