Robert L. Leheny

Bacterial attachment to a fluid interface can lead to the formation of a film with physicochemical properties that evolve with time. We study the time evolution of interface (micro)mechanics for interfaces between oil and bacterial suspensions by following the motion of colloidal probes trapped by capillarity to determine the interface microrheology. Initially, active bacteria at and near the interface drive superdiffusive motion of the colloidal probes. Over timescales of minutes, the bacteria form a viscoelastic film which we discuss as a quasi-two-dimensional, active, glassy system. To study late stage mechanics of the film, we use pendant drop elastometry. The films, grown over tens of hours on oil drops, are expanded and compressed by changing the drop volume. For small strains, by modeling the films as 2D Hookean solids, we estimate the film elastic moduli, finding values similar to those reported in the literature for the bacteria themselves. For large strains, the films are highly hysteretic. Finally, from wrinkles formed on highly compressed drops, we estimate film bending energies. The dramatic restructuring of the interface by such robust films has broad implications, e.g. in the study of active colloids, in understanding the community dynamics of bacteria, and in applied settings including bioremediation.

We report experiments studying the mechanical evolution of layers of the protein lysozyme adsorbing at the air–water interface using passive and active microrheology techniques to investigate the linear and nonlinear rheological response, respectively. Following formation of a new interface, the linear shear rheology, which we interrogate through the Brownian motion of spherical colloids at the interface, becomes viscoelastic with a complex modulus that has approximately power-law frequency dependence. The power-law exponent characterizing this frequency dependence decreases steadily with increasing layer age. Meanwhile, the nonlinear microrheology, probed via the rotational motion of magnetic nanowires at the interface, reveals a layer response characteristic of a shear-thinning power-law fluid with a flow index that decreases with age. We discuss two possible frameworks for understanding this mechanical evolution: gelation and the formation of a soft glass phase.

We employ the Brownian motion of spherical colloids to investigate the microrheology of layers of the protein β-lactoglobulin adsorbing at a decane–water interface. Using an experimental design in which the protein concentration in the aqueous subphase increases in a diffusion-limited manner, we obtain a time-resolved characterization of the interfacial rheology. After undergoing a period of increasing interfacial viscosity, the layers become viscoelastic with a rheology that indicates an approach to a sol–gel transition. This evolution, which depends on subphase pH, contrasts in several significant ways with layer formation by β-lactoglobulin at an air–water interface. Comparison of the diffusivity of colloids of two sizes as the interfacial viscosity increases illustrates the transformation in particle mobility from a three-dimensional character in which the surrounding bulk fluids dominate to a two-dimensional form dictated by the drag from the interfacial layer. This transformation is similarly revealed in the two-point correlations of the particle motion, which show a change in spatial dependence indicating a crossover from three-dimensional to two-dimensional hydrodynamics.

We investigate the mechanical properties of layers of the protein β-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 β-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.

We report x-ray photon correlation spectroscopy and diffusing wave spectroscopy studies of depletion gels formed from nanoscale silica colloids in solutions of nonabsorbing polymer following the cessation of shear. The two techniques provide a quantitatively coherent picture of the dynamics as ballistic or convective motion of colloidal clusters whose internal motion is arrested. While the dynamics possesses features characteristic of nonergodic soft solids, including a relaxation time that grows linearly with the time since shear, comparison with behavior of quenched supercooled liquids indicates that this evolution is not directly related to traditional aging phenomena in glasses.