Interfacial viscoelasticity, yielding and creep ringing of globular protein-surfactant mixtures
Aditya Jaishankar, Vivek Sharma, and Gareth H. McKinley
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 rheologicalmeasurements 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.%). Preferential interfacial adsorption of the mobile surfactant at
these surfactant concentrations results in complete elimination of
the bulk apparent yield stress exhibited by the surfactant-free
BSA solutions.