Annual Report of the Municipal Officers of the Town of Columbia Falls For The Municipal Year 1935-1936

status: publishe

Phase behavior of colloidal dispersions in shear flow

edition: 1ststatus: publishe

Frustrating colloidal rods

The free diffusion of rod-like particles is strongly frustrated by the presence of other rods. This frustration underlies a plethora of phase transitions with increasing concentration of colloidal rods, going from a completely disordered phase (isotropic) to orientational ordering (nematic phase), and to 1-D (smectic phase) and 2-D (columnar phase) positional ordering. We will monitor the dynamic behaviour of the rods throughout this sequence of phase transitions on a single particle level, using fluorescence microscopy. We show how dynamics subtly depends on the specific features of the rods thus elucidating the nature of the complex phase behaviour. Rods can also be frustrated on a more coarse level, as we will see by confining the nematic phase in a range of different geometries. The competition between the elasticity of the nematic phase, wall anchoring and confinement leads to programmable director structures, from which information on the nematic phase can be obtained

Dynamics and colloidal rods at rest and in external fields

Colloidal rods form the most basic system to study the origin of self-assembled structures. Depending on concentration it can form phases with only orientational ordering and 1-, 2- and 3-D positional ordering. In this talk I will first show what can be learned from the dynamical behavior of individual rods about the nature of the equilibrium self-assembly. This is experimentally achieved by tracking rod-like viruses with video fluorescence microscopy. The viruses ideal model systems since they are very slender, monodisperse and can be made with variable stiffness. Dynamic signatures for the different phase transitions are obtained, leading to a reconsideration of the nature of the transitions. Another interesting aspect of colloidal rods is that they are very susceptible to external fields. In shear flow the torque on the rod will cause alignment, while in confinement rods tend to align with the wall. Both effects can drive or frustrate structure formation. In the second part of the talk, I will give several examples of this interplay of rod dispersions with external fields and show pathways to a microscopic understanding of the observed phenomena

The phase behavior of rod-sphere mixtures in equilibrium and under shear flow

We study the phase behavior of rod-sphere mixtures in equilibrium and under shear flow. Mixtures of fd virus, which is an established modelsystem for monodisperse colloidal rods, and density matched polystyrene beads were investigated. We used well defined size ratios L/d around 1, where L=880 nm is the length of the rod-like viruses and d diameter of the density-matched polystyrene beads. Employing Diffuse Wave Light scattering and Confocal microscopy we obtained the equilibrium phase diagram of these mixtures and identified the morphology during phase separation. we identified structure formation and distortion around the gas-liquid phase boundary with a counter-rotating shear cell mounted on the confocal microscope

Time-resolved SANS on wormlike micelles in shear flow

SANS has proven to be a versatile tool for rheologists in achieving their goal of linking micro- or nanoscopic structural and macroscopic flow properties. A nice recent example of the combination rheo-SANS is given by the time-resolved SANS measurements on surfactant wormlike micelles in shear flow. Here we managed to identify the structures of the overstretched system, just before a flow-instability sets in. In principle this technique is applicable to a plethora of systems, but most applications are limited by the scattering intensity. The possibilities of a source like the ESS will be discussed to open up the field of rheo- time-resolved SANS applications