Study Of Complex Fluids Research Articles

Active Stochastic Microrheology using Optical Tweezers

Cells are dynamic structures capable of generating and reacting to physical cues in their environment. Measuring mechanical properties is thus essential for elucidating cell or other material structure-function in particular during dynamic rearrangement of the cytoskeleton. Although a variety of rheological techniques have been developed using video microscopy, AFM, and magnetic traps, the measurable frequency range is limited by the time to obtain the measurement, and forcing conditions such as amplitude, direction, contact geometry, and probe location. Here, we developed active stochastic microrheology using optical tweezers to enhance the temporal resolution and precision of detection. A stochastic force is generated by moving the trap relative to the sample. Both bead displacement and trap position are monitored simultaneously by separate position sensitive devices. With this method, both storage and loss shear moduli of the extracellular matrices can be extracted over a wide frequency range of 10−2 – 103 Hz within a few minutes. Also, this method was used to probe the local mechanical environment of B-cell receptor using antigen specific interaction. We showed that the local mechanical properties are strengthened in response to antigen binding and repeated external excitation in a physiological range of 1–100pN. The mechanical responses can also be measured with respect to direction such as force applied normal and perpendicular to the cell membrane. This technique is useful in characterizing the mechanical properties at a user-defined location and magnitude, over a wide frequency spectrum, in a short time, and with a small deformation < 100nm. With these advantages, the method can also be applied to other cell processes, studies of complex fluids, fibril growth, and polymer solutions. Support from the NIGMS (GM-076689), an NSF Career Award (0643745), and the Singapore-MIT Alliance for Research and Technology (SMART-BioSyM) are gratefully acknowledged.

Computer Simulations: An Overview of Monte Carlo Methods for Fluid Simulation

In computational studies of complex fluids, Monte Carlo simulations play a valuable role. Such simulations can be divided into two classes: those related to efficient data analysis and those aimed at improving the samples' statistical quality.

Thermostat with a local heat-bath coupling for exact energy conservation in dissipative particle dynamics

We present a Markov process which models particle hydrodynamics with conservation of the first three momenta. This is achieved by extending the [Peters, Europhys. Lett. 66, 311 (2004)] and [Lowe, Europhys. Lett. 47, 145 (1999)] method to incorporate energy conservation. The equivalence of the energy conserving Peters method and dissipative particle dynamics with energy conservation (DPDE) in the limit of a vanishing time step is shown. Simple numerical experiments clearly demonstrate the applicability of the methods. This overcomes current limitations of DPDE in the study of complex fluids in the (N,V,E) ensemble.

Combined SAXS−Rheological Studies of Liquid-Crystalline Colloidal Dispersions of Mineral Particles

This article describes simultaneous rheological and small-angle X-ray scattering (SAXS) studies of complex fluids, using a modified rheometer that allows in situ synchrotron SAXS measurements. We investigated the behavior under shear stress of lamellar liquid-crystalline suspensions, recently reported, comprised of covalent mineral sheets of H3Sb3P2O14 in water, that form sols and gels. Two original nematic mineral suspensions of HSbP2O8 disklike and H4Nb6O17 rodlike nanoparticles were also examined. We correlate both the existence of a yield stress and a strong decrease in viscosity (shear-thinning behavior) with textural changes easily detected by SAXS. The exact nature of the phase (nematic or lamellar) does not seem to affect such phenomena as long as there is orientational order. Moreover, strongly flow-birefringent and shear-thinning isotropic suspensions of anisotropic nanoparticles displayed very anisotropic SAXS patterns under shear.

Thermal diffusion in ionic micellar solutions

Thermal diffusion studies of complex fluids may have a deep impact on determining the microscopic origins of the Soret effect, allowing in particular for a clear distinction between single-particle and collective effects. We show for instance that electrostatic interactions have a dramatic influence on the thermal diffusion of charged surfactant micelles. In the dilute regime, the Soret coefficient strongly decreases with increasing solution ionic strength and scales as the square of the Debye-Hückel screening length. Yet, collective effects yield a reversed scenario even at fairly low surfactant concentrations. We find that the single-particle behaviour can be explained using an interfacial tension-driven mechanism originally proposed by Ruckenstein. Interparticle interactions drastically change the Soret coefficient mainly owing to their effects on the solution's osmotic compressibility. The proposed mechanism can be extended to other kinds of solvent-particle interaction and may open up the way to a general picture of thermal diffusion in disperse systems.

A vapor barrier Couette shear cell for small angle neutron scattering measurements

We describe the design and operation of a temperature controlled Couette shear cell for small angle neutron scattering (SANS) studies of complex fluids under flow. This design incorporates a vapor barrier, which prevents sample evaporation to relatively high shear rates. This cell enables the investigation of systems which are highly sensitive to evaporation. Over the duration of a Couette SANS measurement composition phase transitions due to evaporation can be misinterpreted as true shear-induced transformations. We give a brief report of recent experiments performed on one such system: bicontinuous L3 sponge phases for which the surfactant membrane constituents are Cetylpyridinium chloride and hexanol. These clearly demonstrate the limitations of previous designs and the utility of the vapor barrier in measurements of a predicted shear induced sponge to lamellar phase transition. Using this cell we also describe and test a simple and effective way to put SANS data taken in the tangential Couette configuration on an absolute scale.

Imaging Complex Fluids Under Confinement and Flow: Development of Bragg-Fresnel Optics for X-ray Microdiffraction

ABSTRACTWe present results of simultaneous efforts to develop: 1.) Bragg-Fresnel Optics(BFO) to be used in X-ray microdiffraction methods, in particular as applied to structural studies of complex fluids and biomaterials under confinement and flow conditions; and 2.) Methodologies for confining complex fluids and biomaterials for in-situ structural studies. Using microelectronics process technology, we have fabricated linear and circular Bragg-Fresnel Lenses (BFL) in Si and III-V compound semiconductor substrates such as InP, GaAs and GaAs heterostructures with outermost zone width to 0.25 μm. X-ray characterization of linear BFLs were performed on a wiggler beamline at Stanford Synchrotron Radiation Laboratory (SSRL) at x-ray energies of 8 keV and 16 keV. A ∼5 μm focal spot size was obtained with a 50 μm incident beam, which was determined by the partial coherence of the source. On the confinement techniques, we have developed the X-ray Surface Force Apparatus (XSFA) which allows in-situ x-ray diffraction measurements to be made on fluid thin films confined between two atomically smooth surfaces. A new approach is being pursued to study the effects of confinement and flow on complex fluids and biological materials using microchannels fabricated on glass substrates.

The simulation of entropic phase transitions

This paper reviews recent (numerical) progress in the understanding of entropic phase transitions in complex fluids. In particular, I discuss (liquid-)crystal formation and demixing in binary mixtures. In some cases it appears that lessons learnt in the study of complex fluids may have an unexpected relevance for simple fluids.