Biological & Soft Matter Seminar: PhD Seminars

Speaker: Tomer Markovich

Title: Surface Tension of Electrolyte Interfaces: Ionic Specificity within a Field-Theory Approach

Abstract: When salts are added in small quantities to an aqueous solution, its surface tension increases due to the dielectric discontinuity at the air/water surface. This idea was implemented in the pioneering work of Onsager and Samaras, who found a universal limiting law for the dependence of the surface tension on the salt concentration. However, the result implies an increase in the surface tension that is independent of the ion type, which turned out to be violated in many physical realizations.
Employing field-theoretical methods and considering short-range interactions of ions with the surface, we expand the Helmholtz free energy to first-order in a loop expansion and calculate the excess surface tension and to ionic profiles. Our approach is self-consistent and yields an analytical prediction that reunites the Onsager-Samaras pioneering result (which does not agree with experimental data), with the ionic specificity of the Hofmeister series. We obtain analytically the surface-tension dependence on the ionic strength, ionic size and ion-surface interaction, and show consequently that the Onsager-Samaras result is consistent with the one-loop correction beyond mean-field. Our theory fits well a wide range of concentrations for different salts using one fit parameter, reproducing the reverse Hofmeister series for anions at the air/water and oil/water interfaces.

Speaker: Oz Oshri

Title: The effect of strain tensor selection on the elastic theory of thin incompatible sheets

Abstract: We point at inconsistency in the existing theory of incompatible elastic sheets. When applied to compatible sheets, deformed uniaxially by pure bending moments, it generates spurious in-plane stresses. We present an alternative formulation for a class of simple axisymmetric problems. Our theory yields linear, exactly solvable, equations of equilibrium, replacing the non-linear ones derived earlier for these problems. In addition, we obtain a simple criterion determining whether an isometric immersion of such sheets satisfies mechanical equilibrium. When reduced to unidirectional (one-dimensional) deformations the formulation correctly converges to the extensible elastica.

Seminar Organizer: Guy Yaacoby