Combined Passive and Active Microrheology Study of Protein-Layer Formation at an Air−Water Interface

Abstract

We investigate the mechanical properties of layers of the protein beta-lactoglobulin during their formation at the air-water interface using a combination of passive and active microrheological techniques. The passive microrheology, which employs multiple particle tracking measurements using spherical colloids, indicates that the interfacial rheology evolves over time through three stages as protein adsorbs at the interface: (i) an increase in viscosity, (ii) a period of spatial heterogeneity in which the interface contains elastic and viscous regions, and (iii) the development of a uniformly rigid elastic film. Varying solution pH between pH = 5.2, the isoelectric point of beta-lactoglobulin, and pH = 7.0 has no qualitative effect on this mechanical evolution. The active microrheology, which employs ferromagnetic nanowires rotating in response to magnetic torques, similarly shows an increasing interfacial viscosity at early times and evidence of mechanical heterogeneity at intermediate times. However, at late times, the nanowire mobility becomes strongly pH dependent. For pH = 5.2, the layer responds as a rigid elastic film to the stress imposed by the wire. For pH = 7.0, it displays a viscous response that contrasts with the passive measurements. We associate this contrast with a nonlinear response to the wire at late times that reflects a low yield stress of the film at higher pH. This ability to compare passive and active measurements demonstrates the advantage of applying multiple microrheological methods to resolve ambiguity in any single approach.

Monitoring phases and phase transitions in phosphatidylethanolamine monolayers using active interfacial microrheology

“…2, such that the torque balance equation now becomes: $${\mu }_{0}mH\text{sin}\phi =-f_r{\eta }_w{l}^3\frac{d\phi }{dt}+E_{s}true(\frac{\pi }{2}-\phi true)$$ A finite elasticity causes the rod to “stall” $false(\frac{d\phi }{dt}\to 0false)$ at a non-zero angle where the elastic force is balanced by the magnetic torque (note that the magnetic torque goes to zero as φ → 0 in Eqns. 3 and 5) 27, 38, 39 . To detect elasticity, as well as any anisotropic contribution from the monolayer microstructure, torques were applied in both the x and y direction.…”

Particle tracking in drug and gene delivery research: State-of-the-art applications and methods

“…In a more complex, heterogeneous system like CF sputum, the individual particle MSDs can vary significantly (Fig. 9B), and there is not necessarily a single value that can describe the entire ensemble [57, 126, 127]. …”

“…In some systems [126, 128] the MSDs of individual particles are well separated into two or more modes, such as a group of diffusive particles and a group of immobilized particles with MSDs that quickly asymptote to a fixed value. Once the groups are identified, the particles within each group can be averaged.…”

Surface shear inviscidity of soluble surfactants

“…3), giving η S;app = 4ηa 3π ; [5] corresponding to η S,app = 0.0042 μN·s/m for a 10-μm probe on a water subphase. This sensitivity limit implies that O(0.01 μN·s/m) surface shear viscosities can be resolvable unambiguously, whereas smaller surface shear viscosities would change the relative resistance ζ R /ζ R,clean by O (1) (36,37) and passive (colloid-tracking) microrheology (38,39) have nominally higher sensitivity O(0.001 μN·s/m), they excite mixed rheological deformations (i.e., shear, extension, and compression), and thus combine multiple distinct properties into one measured quantity. It is very difficult to extract η S from rheologically mixed flows, especially in the Bo ∼ 1 limit.…”