Linear and nonlinear microrheometry of small samples and interfaces using microfabricated probes

Abstract

We describe a microrheological strategy that enables sensitive surface shear rheology measurements of surfactant-laden interfaces, with the capacity to simultaneously visualize deforming interfaces. This technique utilizes a ferromagnetic microbutton probe pinned to a fluid-fluid interface, and actively torqued or forced with externally controlled electromagnets. Various modes of operation are possible: Smallamplitude oscillatory rotations, which provide frequency-dependent viscoelastic shear moduli; controlled torque (analogous to fixing shear stress); controlled rotation rate (analogous to fixing strain rate); and imposed force (analogous to active, translational microrheology). The circular shape of the probe ensures pure shear strains (when driven to rotate). We describe the experimental apparatus, its measurement limits and sources of error. We then highlight its versatility and capabilities with measurements on a variety of qualitatively distinct systems, including purely viscous monolayers, block-copolymer interfaces, aging and evolving interfaces, colloidal monolayers, and bulk rheometry of Newtonian and viscoelastic materials, with sample volumes as small as 2 ll. V

Isostructural solid–solid phase transition in monolayers of soft core–shell particles at fluid interfaces: structure and mechanics

“…The single coil (b in Figure 2) was used to align the magnetic moment of the microdisk perpendicular to the other coils (a and c in Figure 2), which were connected in series and used to generate the oscillating magnetic field to apply shear to the interface. The full details of the experimental setup can be found elsewhere 50 ; for the purpose of these experiments, the characteristic response time of the magnets was below 1 ms, significantly smaller than the range of frequencies applied in our measurements (0.1-3 Hz). …”

“…rheometer [48][49][50] , we obtain a direct link between the interface microstructure and its shear rheology. Interestingly, we discovered the occurrence of an isostructural solid-solid phase transition in the monolayer between two different hexagonal phases, one where the shells of the particles are in contact and one where the cores enter into physical contact.…”

“…After spreading the microgels at the interface, a 100 µm diameter amphiphilic magnetic disk was inserted at the interface inside the circular aperture and was externally manipulated by means of the electromagnets. The disk could be rotated at the interface in an oscillatory fashion by applying oscillatory fields of known frequency and amplitude, creating a 2D miniaturized version of a 2D large-gap Couette rheometer 50 . Similar active microrheology instruments have been successfully used to measure the interfacial rheology of colloidal and lipid monolayers at * As a side note, depositions are more effectively carried out from water/nhexane interfaces to minimize the effects of capillary forces during drying 55 .…”

Pressure-dependent surface viscosity and its surprising consequences in interfacial lubrication flows

“…The width of the fluid gap is assumed to be much smaller than the linear dimension of the particles that confine the flow to the gap, so that the approximations of lubrication theory hold. These examples, besides being within the reach of established experimental techniques [8,12], will also serve as building blocks towards developing the physical intuition and the mathematical machinery behind suspension-level interactions. We shall consider two representative cases, both involving two surfaces separated by a thin gap and approaching each other or sliding past one another.…”

Effect of Ethylcellulose on the Rheology and Mechanical Heterogeneity of Asphaltene Films at the Oil–Water Interface

“…The magnetic microbutton microrheometry technique and the custom water-oil sample cell for interfacial rheology are described in previous publications 17,18,45,50,51 and are summarized here. The magnetic moment of the microbutton is calibrated following the method described previously.…”