M. Paul Lettinga

A sequence of physical processes determined and quantified in large-amplitude oscillatory shear (LAOS): Application to theoretical nonlinear models

The nonlinear yielding responses of three theoretical models, including the Bingham, a modified Bingham, and Giesekus models, to large-amplitude oscillatory shear are investigated under the framework proposed recently by Rogers et al. (2011). Under this framework, basis states are allowed to wax and wane throughout an oscillation, an approach that conflicts directly with the assumptions of all Fourier-like linear algebraic approaches. More physical yielding descriptions of the nonlinear waveforms are attained by viewing the responses as representing purely elastic to purely viscous sequences of physical processes. These interpretations are compared with, and contrasted with, results obtained from linear algebraic analysis methods: Fourier-transform rheology; and the Chebyshev description of the so-called elastic and viscous stress components σ′ and σ″. Further, we show that the discrepancies between the built-in model responses and parameters, and the interpretations of the Chebyshev and Fourier coefficie...

Self-diffusion of rodlike viruses through smectic layers.

We report the direct visualization at the scale of single particles of mass transport between smectic layers, also called permeation, in a suspension of rodlike viruses. Self-diffusion takes place preferentially in the direction normal to the smectic layers, and occurs by quasiquantized steps of one rod length. The diffusion rate corresponds with the rate calculated from the diffusion in the nematic state with a lamellar periodic ordering potential that is obtained experimentally.

Reversible Gelation of Rod-Like Viruses Grafted with Thermoresponsive Polymers

The synthesis and selected macroscopic properties of a new model system consisting of poly(N-isopropylacrylamide) (PNIPAM)-coated rod-like fd virus particles are presented. The sticky rod-like colloids can be used to study effect of particle shape on gelation transition, the structure and viscoelasticity of isotropic and nematic gels, and to make both open isotropic as well as ordered nematic particle networks. This model system of rod-like colloids, for which the strength of attraction between the particles is tunable, is obtained by chemically grafting highly monodisperse rod-like fd virus particles with thermoresponsive polymers, e.g. PNIPAM. At room temperature, suspensions of the resulting hybrid PNIPAM-fd are fluid sols which are in isotropic or liquid crystalline phases, depending on the particle concentration and ionic strength. During heating/cooling, the suspensions change reversibly between sol and gel state near a critical temperature of approximately 32 degrees C, close to the lower critical solution temperature of free PNIPAM. The so-called nematic gel, which exhibits a cholesteric feature, can therefore be easily obtained. The gelation behavior of PNIPAM-fd system and the structure of the nematic gel have been characterized by rheology, optical microscopy and small-angle X-ray scattering.

Dynamical and structural insights into the smectic phase of rod-like particles

Self-diffusion in a model system of rod-like particles is studied in the smectic (or lamellar) phase. The experimental system is formed by a colloidal suspension of filamentous fd virus particles, which allows the direct visualization at the scale of the single particle of mass transport between the smectic layers. Self-diffusion takes place preferentially in the direction normal to the smectic layers and occurs in steps of one rod length, reminiscent of a hopping-type of transport. The probability density function is obtained experimentally at different times and is found to be in qualitative agreement with theoretical predictions based on a dynamical density functional theory.

Thermal diffusion of a stiff rod-like mutant Y21M fd-virus.

We investigated the thermal diffusion phenomena of a rodlike mutant filamentous fd-Y21M virus in the isotropic phase by means of an improved infrared thermal-diffusion-forced Rayleigh scattering (IR-TDFRS) setup optimized for measurements of slowly diffusing systems. Because this is the first thermal diffusion study of a stiff anisotropic solute, we investigate the influence of the shape anisotropy on the thermal diffusion behavior. The influence of temperature, fd-Y21M concentration, and ionic strength in relation with the thermodiffusion properties is discussed. We characterize and eliminate the effect of these parameters on the absolute diffusion of the rods and show that diffusion determines the behavior of the Soret coefficient because the thermal diffusion coefficient is constant in the investigated regime. Our results indicate that for the thermal diffusion behavior structural changes of the surrounding water are more important than structural changes between the charged macroions. In the investigated temperature and concentration range, the fd-Y21M virus is thermophobic for the low salt content, whereas the solutions with the high salt content change from thermophobic to thermophilic behavior with decreasing temperature. A comparison with recent measurements of other charged soft and biological matter systems shows that the shape anisotropy of the fd-virus becomes not visible in the results.

Probing structure in colloidal gels of thermoreversible rodlike virus particles: Rheology and scattering

Aggregated suspensions of rodlike particles are commonly encountered in soft biological materials and their solidlike response at extremely low volume fractions is also exploited technologically. Understanding the link between the physicochemical parameters such as size, aspect ratio, volume fraction, and interparticle forces with the resulting microstructure and the subsequent rheological response remains challenging. In the present work, suspensions of monodisperse rodlike virus particles, whose surface is modified by grafting with a thermoreversible polymer poly(N-isopropylacrylamide), are used as a model system. The repulsive and attractive contributions to the total interaction potential can be changed independently by varying the ionic strength and the temperature. The effects of these changes on the strength and structure of gels have been studied near the gel transition using a combination of rheological and scattering measurements. Rheological measurements of the near critical gel properties as a...

Dynamics in the smectic phase of stiff viral rods

We report on the dynamics in colloidal suspensions of stiff viral rods, called fd-Y21M. This mutant filamentous virus exhibits a persistence length 3.5 times larger than the wild-type fd-wt. Such a virus system can be used as a model system of rodlike particles for studying their self-diffusion. In this paper, the physical features, such as rod contour length and polydispersity have been determined for both viruses. The effect of viral rod flexibility on the location of the nematic-smectic phase transition has been investigated, with a focus on the underlying dynamics studied more specifically in the smectic phase. Direct visualization of the stiff fd-Y21M at the scale of a single particle has shown the mass transport between adjacent smectic layers, as found earlier for the more flexible rods. We could relate this hindered diffusion with the smectic ordering potentials for varying rod concentrations. The self-diffusion within the layers is far more pronounced for the stiff rods as compared to the more flexible fd-wt viral rod.

Nematic-isotropic spinodal decomposition kinetics of rodlike viruses

We investigate spinodal decomposition kinetics of an initially nematic dispersion of rodlike viruses. Quench experiments are performed from a flow-stabilized homogeneous nematic state at a high shear rate into the two-phase isotropic-nematic coexistence region at a zero shear rate. We present experimental evidence that spinodal decomposition is driven by orientational diffusion, in accordance with a very recent theory.

Kinetic pathways of the nematic-isotropic phase transition as studied by confocal microscopy on rod-like viruses

We investigate the kinetics of phase separation for a mixture of rod-like viruses (fd) and polymer (dextran), which effectively constitutes a system of attractive rods. This dispersion is quenched from a flow-induced fully nematic state into the region where the nematic and the isotropic phase coexist. We show experimental evidence that the kinetic pathway depends on the overall concentration. When the quench is made at high concentrations, the system is meta-stable and we observe typical nucleation-and-growth. For quenches at low concentration the system is unstable and the system undergoes a spinodal decomposition. At intermediate concentrations we see the transition between both demixing processes, where we locate the spinodal point.

Glasses of dynamically asymmetric binary colloidal mixtures: Quiescent properties and dynamics under shear

We investigate mixing effects on the glass state of binary colloidal hard-sphere-like mixtures with large size asymmetry, at a constant volume fraction phi = 0.61. The structure, dynamics and viscoelastic response as a function of mixing ratio reflect a transition between caging by one or the other component. The strongest effect of mixing is observed in systems dominated by caging of the large component. The possibility to pack a large number of small spheres in the free volume left by the large ones induces a pronounced deformation of the cage of the large spheres, which become increasingly delocalised. This results in faster dynamics and a strong reduction of the elastic modulus. When the relative volume fraction of small spheres exceeds that of large spheres, the small particles start to form their own cages, slowing down the dynamics and increasing the elastic modulus of the system. The large spheres become the minority and act as an impurity in the ordering beyond the first neighbour shell, i.e. the cage, and do not directly affect the particle organisation on the cage level. In such a system, when shear at constant rate is applied, melting of the glass is observed due to facilitated out-of-cage diffusion which is associated with structural anisotropy induced by shear.