Oscillating Bubble SHG on Surface Elastic and Surface Viscoelastic Systems:  New Insights in the Dynamics of Adsorption Layers

Abstract

Surface rheology governs a great variety of interfacial phenomena such as foams or emulsions and plays a dominant role in several technological processes such as high-speed coating. Its major difference with bulk rheology resides in the high compressibility of the surface phase, which is the direct consequence of the molecular exchange between adsorbed and dissolved species. In analogy to bulk rheology, a complex surface dilational modulus, epsilon, which captures surface tension changes upon defined area changes of the surface layer, can be defined. The module epsilon is complex, and the molecular interpretation of the dissipative process that gives rise to the imaginary part of the module is subject to some controversy. In this contribution, we used the oscillating bubble technique to study the surface dilational modulus in the mid-frequency range. The dynamic state of the surface layer was monitored by a pressure sensor and by surface second-harmonic generation (SHG). The pressure sensor measures the real and imaginary part of the modulus while SHG monitors independently the surface composition under dynamic conditions. The experiment allows the assessment of the contribution of the compositional term to the surface dilational modulus epsilon. Two aqueous surfactant solutions have been characterized: a surface elastic and a surface viscoelastic solution. The elastic surface layer can be described within the framework of the extended Lucassen-van den Tempel Hansen model. The change in surface concentration is in phase with the relative area change of the surface layer, which is in strong contrast with the results obtained from the surface viscoelastic solution. Here, surface tension, area change, and surface composition are phase-shifted, providing evidence for a nonequilibrium state within the surface phase. The data are used to assess existing surface rheology models.

The Link between Ion Specific Bubble Coalescence and Hofmeister Effects Is the Partitioning of Ions within the Interface

“…Slower equilibration between a sublayer and a surface layer is associated with dilational surface viscosity and longer film lifetimes, in ionic surfactant solutions. 22 We suggest that a similar surface rheology may be present in electrolyte solutions. The model of surface-subsurface equilibration is consistent with the observed difference in inhibition between RR and ββ, and Rβ and βR electrolytes.…”

Impact of the Imaginary Part of the Surface Dilatational Modulus on the Splashing Behavior of Drops

“…12 According to the functional dependence of the phase ϕ of the complex modulus E at higher frequencies, adsorption layers can be classified as either surface elastic or surface viscoelastic. 13 The expansion or compression of the surface creates a nonequilibrium state. As a result, the surface coverage deviates from the equilibrium value.…”

“…Viscous effects at lower frequencies are solely caused by bulk diffusion. 13 Adsorption layers are classified as surface viscoelastic if the phase does not vanish at higher frequencies and the magnitude of the dilatational modulus increases with the frequency of the bubble oscillation. The surface layer exhibits surface viscoelastic behavior at all frequencies, implying that energy is dissipated within the surface layer.…”

Comparison of the Surface Dilational Properties of Block Copolymers and Sodium Oleate Using the Oscillating Bubble Method

“…[3] Therefore, the dynamic dilational properties on the adsorption layer of surfactant have been set store by many investigators in recent years. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Amphiphilic block copolymers have attracted widespread interest for both scientific and practical reasons. [21] These macromolecules adsorb at various interfaces and play a significant role in dispersion stabilization, foaming, and emulsification.…”