Allan Raudsepp

A rheo-optical study of shear rate and optical anisotropy in wormlike micelles solutions

The flow behaviour of wormlike micelles solutions composed of the surfactant cetylpyridinium chloride (CPCl) and counter-ion sodium salicylate (NaSal) at a molar ratio [CPCl]/[NaSal] = 2 in brine [NaCl] = 0.5 M in a cylindrical Couette geometry was examined using homodyne PCS and ellipsometry. Homodyne PCS was used to profile local shear rate and ellipsometry to concurrently profile local optical anisotropy of the fluid. Shear thinning was observed and was correlated to an increase in turbidity and a breakdown in the stress-optic law. A stress plateau observed in mechanical measurements was correlated with the partitioning of the fluid into regions of low shear rate/low turbidity/high birefringence and high shear rate/high turbidity/low birefringence. The partitioning observed was inconsistent with a simple interpretation of the lever rule.

DNA visualization in single molecule studies carried out with optical tweezers: Covalent versus non-covalent attachment of fluorophores.

In this study, we investigated the use of the covalent attachment of fluorescent dyes to double-stranded DNA (dsDNA) stretched between particles using optical tweezers (OT) and compared the mechanical properties of the covalently-functionalized chain to that of unmodified DNA and to DNA bound to a previously uncharacterized groove-binder, SYBR-gold. Modified DNA species were obtained by covalently linking azide-functionalized organic fluorophores onto the backbone of DNA chains via the alkyne moieties of modified bases that were incorporated during PCR. These DNA molecules were then constructed into dumbbells by attaching polystyrene particles to the respective chain ends via biotin or digoxigenin handles that had been pre-attached to the PCR primers which formed the ends of the synthesized molecule. Using the optical tweezers, the DNA was stretched by separating the two optically trapped polystyrene particles. Displacements of the particles were measured in 3D using an interpolation-based normalized cross-correlation method and force-extension curves were calculated and fitted to the worm-like chain model to parameterize the mechanical properties of the DNA. Results showed that both the contour and persistence length of the covalently-modified dsDNAs were indistinguishable from that of the unmodified dsDNA, whereas SYBR-gold binding perturbed the contour length of the chain in a force-dependent manner.

A study of the nonlinear rheology of complex fluids using diffusing wave spectroscopy

Diffusing wave spectroscopy’s (DWS) sensitivity to local shear rate has been used to investigate the nonlinear rheology of (1) a doped shear thinning, semidilute polymer solution measured in the cone-plate and parallel-plate geometries and of (2) doped shear banding wormlike micelles solutions measured in the cylindrical-Couette geometry. DWS measurements of the semidilute polymer solution in the cone-plate geometry under stress controlled conditions were used to construct a flow curve for the material, the expected radial dependence of the local shear rate being demonstrated in the parallel-plate geometry. The method is shown to be sensitive to shear banding in semidilute wormlike micelles in the cylindrical-Couette geometry under strain controlled conditions, and DWS correlation functions were quantitatively interpreted using a heterogeneous shear flow model. Flow curves and shear rate profiles were inferred from these measurements.

Developing a video tracking method to study interactions between close pairs of optically trapped particles in three dimensions

We develop a video tracking method that utilizes an interpolation-based normalized cross-correlation approach to track the position of microscopic spherical particles in three dimensions. Subnanometer resolution is demonstrated. The method does not assume that the particles image is radially symmetric, making it useful for determining the position when particles are close and their images overlap. This is demonstrated in a study of the electrostatic and hydrodynamic interactions between a pair of beads in dual laser tweezers traps.

High frequency linear rheology of complex fluids measured from their surface thermal fluctuations

We characterize the linear viscoelastic properties of complex fluids using a new technique, based on the measurement of surface fluctuations: surface fluctuation specular reflection (SFSR) spectroscopy. The thermally excited waves propagating on a free surface are measured through the deflection of a laser beam specularly reflected from that surface. Elastic and loss moduli of the complex fluids are inferred from the measured power spectrum density of thermal noise, with the implicit use of Kramers–Kronig relations. The technique, besides being noninvasive, provides rheological data in a large frequency range and at vanishing strains. It is therefore especially well suited for the rheological characterization of complex fluids. We present measurements of the viscoelastic modulus of supramolecular polymer solutions in a frequency range extending up to six decades. We compare the SFSR measurements with rheometric data at low and high frequencies, and show that they are in good agreement. Using supramolecular polymer solutions of different natures, exhibiting or not surface viscoelasticity, we furthermore show that SFSR provides a characterization of the bulk properties of the fluids. In addition, we discuss the accuracy of the measurements.

Helium ion implantation in SiAlON: Characterisation of cavity structures using TEM and IBA

Abstract Highly swollen nanoporous layers produced in material surfaces by He implantation are of special interest for applications such as catalysis. Here we investigate whether nanoporous layers can be produced in the covalently bonded insulating ceramic, SiAlON. The retention of highly swollen porous structures in thinned TEM sections prepared from such hard brittle materials is particularly challenging. We have successfully prepared such sections both parallel to, and perpendicular to, the implanted surface. At intermediate doses the bubble structures are very similar to those found in metals. At high helium doses local swellings at depths around the mean projected range of the He ions (∼360 nm) are estimated to be well in excess of 200%. Bubble structures are stable under heating to temperatures up to 1200°C. It is found that the highly cavitated layer is buried below a crystalline overlayer of compact SiAlON. This overlayer is sufficiently compact to resist the diffusion of gold from a surface layer to the cavities below.

Measuring the interaction between a pair of emulsion droplets using dual-trap optical tweezers

Optical tweezers have been used to investigate the dependance of electrostatic inter-particle forces on separation, in systems consisting of pairs of either model silica beads or emulsion droplets. Measurements were carried out as a function of ionic strength and, at salt concentrations where the Debye length was larger than the standard deviation of Brownian fluctuations of the particles in the traps, results were found to agree reasonably well with the predictions of DLVO theory. Experiments were also carried out where the salt concentration of the environment was changed in real-time while interactions were continuously measured. Specifically, single pairs of particles or emulsion droplets were held in a microfluidic channel in close proximity to an interface created between milliQ water and a 5 mM NaCl solution. Changes in the force–separation curves were measured as a function of time and used to monitor changes in the Debye length, and thus the local salt concentration, as ions diffused away from the interface. The results were shown to be consistent with expectations based on a relevant diffusion equation.

Two beam surface fluctuation specular reflection spectroscopy

In surface fluctuation specular reflection spectroscopy (SFSRS) deflections of a specularly reflected laser beam are used to characterize thermally excited surface waves. Here we report on a new two beam version of SFSRS in which the deflections of two reflected laser beams from separate locations on a surface are correlated. We demonstrate that this new two beam SFSRS technique can be used to determine directly the power spectrum of height fluctuation of thermally excited surface waves over a large range of both frequencies and wavevectors. In addition, we show that the technique is well suited for materials ranging from simple liquids to complex liquids and soft solids, including turbid materials.

Mapping the restoring force of an optical trap in three dimensions using a two laser dual-trap approach

A two laser optical tweezers set-up is developed and used to measure deflections of a microsphere trapped in a calibrated spatial light modulator steered probe trap as it is stepped through a three dimensional grid about a fixed test trap. These measurements are used to map the restoring force of the test trap on the microsphere in three dimensions. Results are validated over a common range by comparison to drag force measurements for both silica and polystyrene microspheres.

Multidimensional mapping of the restoring force of an optical trap using triangular wave flow

A method for mapping the spatial dependence of the restoring force of an optical trap in multiple dimensions using triangular wave flow is considered. A theoretical description of the transient motion of an optically trapped massless microsphere in triangular wave flow is introduced and verified experimentally. Guided by theory, the method is first demonstrated using one-dimensional (1D) triangular wave flow to measure the transverse and axial dependence of the restoring force of an optical trap for polystyrene microspheres with diameters from 360 to 3000 nm and is compared to more conventional measurements. While the experimental results are self-consistent, the expected size dependence predicted by theory Nieminensps2007 is not observed for the optical traps examined here. Two-dimensional (2D) triangular-wave flow is then used to the measure the spatial dependence of the restoring force in the transverse–axial plane. The resulting map shows that a spatial asymmetry, not predicted by theory, is present in the optical force field.