High frequency viscoelastic measurements using optical tweezers on wormlike micelles of nonionic and cationic surfactants in aqueous solutions

Ken Morishima and Tadashi Inoue

Linear viscoelasticity of two kinds of wormlike micelles in aqueous solutions, one is nonionic surfactants and the other one is cationic surfactant with organic salt, was measured over a wide frequency range with Brownian motion tracking microrheology (BMTR) using optical tweezers and a conventional rheometer. In BMTR measurements, the Brownian motion of a small particle embodied in the sample is traced and the complex modulus is calculated from the trajectory. The wideband linear viscoelastic spectra thus obtained for each of wormlike micelles were classified into the following three relaxation types as already known. Type A is similar to the spectrum of the nonentangle polymer solutions, type B similar to that of the entanglepolymer systems, and type C has a single Maxwell relaxation at low frequencies. In the high-frequency region, spectra of all types showed a common power-law relaxation, which reflects the reorientation of the viscoelastic segment of the wormlike micelles, indicating that the dynamics of wormlike micelles is identical with that of the ordinary polymeric systems. For type A solutions, the molar mass of wormlike micelles was estimated by fitting the beads-spring models to the viscoelastic spectra. For type B solutions, the molar mass was estimated by using the universality of entangled system. For the case of nonionic micelles, thus determined molar mass is in good agreement with the reported result with the light scattering measurement.

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Bond rupture between colloidal particles with a depletion interaction

Kathryn A. Whitaker and Eric M. Furst

The force required to break the bonds of a depletion gel is measured by dynamically loading pairs of colloidal particles suspended in a solution of a nonadsorbing polymer. Sterically stabilized poly(methyl methacrylate) colloids that are 2.7 μm diameter are brought into contact in a solvent mixture of cyclohexane-cyclohexyl bromide and polystyrene polymer depletant. The particle pairs are subject to a tensile load at a constant loading rate over many approach-retraction cycles. The stochastic nature of the thermal rupture events results in a distribution of bond rupture forces with an average magnitude and variance that increases with increasing depletant concentration. The measured force distribution is described by the flux of particle pairs sampling the energy barrier of the bond interaction potential based on the Asakura–Oosawa depletion model. A transition state model demonstrates the significance of lubricationhydrodynamic interactions and the effect of the applied loading rate on the rupture force of bonds in a depletion gel.

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A model colloidal gel for coordinated measurements of force, structure, and rheology

Lilian C. Hsiao, Michael J. Solomon, Kathryn A. Whitaker and Eric M. Furst

We introduce a model gel system in which colloidal forces, structure, and rheology are measured by balancing the requirements of rheological and microscopy techniques with those of optical tweezers. Sterically stabilized poly(methyl methacrylate) colloids are suspended in cyclohexane (CH) and cyclohexyl bromide (CHB) with dilute polystyrene serving as a depletion agent. A study of the optical trap strength, rheology, and microscopic structure of the gels as a function of CH/CHB solvent composition identifies the conditions for which these measurements can be applied to characterize gel properties. The results indicate that a solvent comprising 37% weight fraction CH (wCH  = 0.37) provides sufficient refractive index contrast to enable optical trapping, while maintaining good confocal microscopy imaging quality and minimal sedimentation effects on the bulk rheology. At this condition, and at a depletant concentration c = 8.64 mg/ml (c/c* = 0.81), results from optical trapping in a dilute sample show that 50% of bonds rupture at (3.3 ± 0.5) pN. The linear strain-dependent elastic modulus of the corresponding gel (ϕ = 0.20) is G′ = (1.8 ± 0.6) Pa, and the mean contact number of the colloids in the gel structure is 〈z〉 = 5.4 ± 0.1. The development of this model colloidal gel system yields a concomitant characterization of the interparticle forces, microstructure, and bulk rheology in a single experimental system, thereby allowing the simultaneous comparison of these different measures.

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