Interfacial viscoelasticity, yielding and creep ringing of globular protein–surfactant mixtures

Abstract

Protein-surfactant mixtures arise in many industrial and biological systems, and indeed, blood itself is a mixture of serum albumins along with various other surface-active components. Bovine serum albumin (BSA) solutions, and globular proteins in general, exhibit an apparent yield stress in bulk rheological measurements at surprisingly low concentrations. By contrasting interfacial rheological measurements with corresponding interface-free data obtained using a microfluidic rheometer, we show that the apparent yield stress exhibited by these solutions arises from the presence of a viscoelastic layer formed due to the adsorption of protein molecules at the air-water interface. The coupling between instrument inertia and surface elasticity in a controlled stress device also results in a distinctive damped oscillatory strain response during creep experiments known as"creep ringing". We show that this response can be exploited to extract the interfacial storage and loss moduli of the protein interface. The interfacial creep response at small strains can be described by a simple second order system, such as the linear Jeffreys model, however the interfacial response rapidly becomes nonlinear beyond strains of order 1%. We use the two complementary techniques of interfacial rheometry and microfluidic rheometry to examine the systematic changes in the surface and bulk material functions for mixtures of a common non-ionic surfactant, Polysorbate 80, and BSA. It is observed that the nonlinear viscoelastic properties of the interface are significantly suppressed by the presence of even a relatively small amount of surfactant (c surf > 10 −3 wt.%).

Power-law rheology in the bulk and at the interface: quasi-properties and fractional constitutive equations

“…We have shown in a previous study that this inertio-elastic phenomenon can be observed not just in the bulk, but at interfaces as well [6]. These periodic oscillations decay exponentially with time due to viscous dissipation, and this phenomenon is often termed creep ringing.…”

“…Although the presence of these oscillations is generally regarded as an intrusion, these transients can, in fact, be exploited to extract useful information about the linear viscoelasticity of soft materials [59,60]. In previous work, using BSA solutions exhibiting interfacial viscoelasticity [6], we have shown that this technique of extracting interfacial properties even presents certain advantages over the conventional technique of conducting frequency sweep measurements to high frequencies. In this earlier study, we also noted that solutions of BSA exhibit a power-law creep response at long times, which could not be adequately captured with the linear Maxwell-Jeffreys model that was considered analytically.…”

Surface shear inviscidity of soluble surfactants

“…Although not exhaustive, the wide range of soluble, relatively low M w surfactant species tested here (SI Appendix, Table S1)--including ionic, nonionic, and polymeric surfactants of high-and low-foaming character--strongly supports the hypothesis that soluble surfactants generically exhibit extremely small surface shear viscosities. Competitive adsorption studies indirectly support this hypothesis, as the addition of soluble surfactants typically reduce the measured interfacial shear moduli of adsorbed protein layers [BSA with Tween 80 (65,66), beta-Lactoglobulin with SDS (58), Tween 20 (67), and pluronic F-127 (68) and biofilms with Tween 20 (69)].…”

Rheological behavior of human blood in uniaxial extensional flow

“…Copley and King [38] showed that viscoelastic effects found in plasma are attributed to the surface layer plasma protein at the liquid-air interface. More recently, Jaishankar et al [39] found that viscoelastic effects of bovine serum albumin solutions in shear flows arise from the presence of a viscoelastic layer, which results from the adsorption of protein also at the liquid-air interface. In this regard, Brust et al [19] also performed measurements of the extensional rheology of plasma with capillary bridges surrounded by silicone oil in order to ensure that the viscoelastic behavior found for human plasma are not due to the viscoelastic layer formed by the adsorption of protein molecules at the air-liquid interface.…”