Naveen Krishna Reddy

Flow Dichroism as a Reliable Method to Measure the Hydrodynamic Aspect Ratio of Gold Nanoparticles

Particle shape plays an important role in controlling the optical, magnetic, and mechanical properties of nanoparticle suspensions as well as nanocomposites. However, characterizing the size, shape, and the associated polydispersity of nanoparticles is not straightforward. Electron microscopy provides an accurate measurement of the geometric properties, but sample preparation can be laborious, and to obtain statistically relevant data many particles need to be analyzed separately. Moreover, when the particles are suspended in a fluid, it is important to measure their hydrodynamic properties, as they determine aspects such as diffusion and the rheological behavior of suspensions. Methods that evaluate the dynamics of nanoparticles such as light scattering and rheo-optical methods accurately provide these hydrodynamic properties, but do necessitate a sufficient optical response. In the present work, three different methods for characterizing nonspherical gold nanoparticles are critically compared, especially taking into account the complex optical response of these particles. The different methods are evaluated in terms of their versatility to asses size, shape, and polydispersity. Among these, the rheo-optical technique is shown to be the most reliable method to obtain hydrodynamic aspect ratio and polydispersity for nonspherical gold nanoparticles for two reasons. First, the use of the evolution of the orientation angle makes effects of polydispersity less important. Second, the use of an external flow field gives a mathematically more robust relation between particle motion and aspect ratio, especially for particles with relatively small aspect ratios.

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...

Anisotropy of nonaqueous layered silicate suspensions subjected to shear flow

Nonaqueous layered silicate suspensions have a complex rheological behavior due to the presence of a microstructure on multiple length scales, which is sensitive to flow and flow history. In the present work, the flow-induced orientation and anisotropy of the nonequilibrium metastable structures in nonaqueous layered silicate suspensions has been studied using a combination of light scattering, scattering dichroism, and advanced rheometric measurements, including two dimensional small amplitude oscillatory shear (2D-SAOS) flow experiments. The nature of the structures during flow was mainly studied by means of small angle light scattering patterns. Linear dichroism measurements in the vorticity and velocity gradient directions were used to assess the microstructural anisotropy. The changes observed in the vorticity plane developed in the same range of shear rate as the shear-thinning behavior of the suspensions. Scattering dichroism was used to demonstrate that the flow-induced anisotropy was locked in up...

Flow-induced conformational changes in gelatin structure and colloidal stabilization.

Flow can change the rate at which solutes adsorb on surfaces by changing mass transfer to the surface, but moreover, flow can induce changes in the conformation of macromolecules in solution by providing sufficient stresses to perturb the segmental distribution function. However, there are few studies where the effect of flow on macromolecules has been shown to alter the structure of macromolecules adsorbed on surfaces. We have studied how the local energy dissipation alters the adsorption of gelatin onto polystyrene nanoparticles ( r = 85 nm). The change in the nature of the adsorbed layer is manifest in the change in the ability of the nanoparticles to resist aggregation. Circular dichroism spectroscopy was used to assess conformational changes in gelatin, and dynamic light scattering was used to assess the colloid stability. Experiments were conducted in a vortex jet mixer where energy density and mixing times have been quantified; mixing of the gelatin and unstable nanoparticles occurs on the order of milliseconds. The adsorption of the gelatin provides steric stabilization to the nanoparticles. We found that the stability of the gelatin-adsorbed nanoparticles increased with increasing mixing velocities: when the mixing velocities were changed from 0.9 to 550 m/s, the radius of the nanoclusters (aggregates) formed 12 h after the mixing decreased from 2620 to 600 nm. Increasing temperature also gave rise to similar trends in the stability behavior with increasing temperature, leading to increasing colloid stability. Linear flow birefringence studies also suggested that the velocity fields in the mixer are sufficiently strong to produce conformational changes in the gelatin. These results suggest that the energy dissipation produced by mixing can activate conformational changes in gelatin to alter its adsorption on the surfaces of nanoparticles. Understanding how such conformational changes in gelatin can be driven by local fluid mechanics and how these changes are related to the adsorption behavior of gelatin is very important both industrially and scientifically.

Rheo-optical response of carbon nanotube suspensions

In this work, the rheo-optical response of multiwalled carbon nanotube (MWCNT) suspensions was analyzed. Dichroism was obtained using a polarization-modulation technique in parallel disks and for the first time for these particles in a Couette flow geometry. MWCNTs were dispersed in a Newtonian epoxy matrix, at different concentrations covering the dilute and semidilute regimes. Measurements of dichroism were performed as functions of shear rate and nanotube concentration. Surprisingly, the ultimate average orientation angle with respect to the flow direction was far from zero degree, even at high Peclet (Pe) numbers in very dilute suspensions. To explain this peculiar behavior, a new model for flexible rods, valid in the dilute regime, is proposed. It is based on the development of Strautins and Latz [Rheol. Acta 46, 1057–1064 (2007)] that considers flexible rods made of beads and connectors. We modified their bending potential that allows only straight rods at equilibrium with a harmonic cosine expressi...

Self-propelling micro-disks

In this paper we introduce a simple and scalable method to produce micrometer sized ‘Janus’ disks whose rim is coated half with platinum/palladium and half with gold. The disks pinned upright to the air/liquid interface exhibit self-propulsion of ∼100 μm/s when submerged in H2O2 solution, due to the catalytic growth of oxygen bubbles at the disks upper (platinum/palladium-coated) rim. The disks exhibit two different travel trajectories, linear and rotary, depending on the bubble growth position, and are propelled via two different mechanisms, the bubble growth and the bubble burst. The displacement speed due to the bubble burst is three orders of magnitude larger than from the bubble growth process, whereas displacement distances are of similar order of magnitude for both processes.

Metallic and bi-metallic Janus nanofibers: electrical and self-propulsion properties

Aligned polymeric nanofibers obtained via electrospinning were sputter-coated with metals on both their front and back sides. When both sides were coated with the same metal (gold, silver or platinum/palladium alloy), the nanofibers behaved as pure conductors. However, coating front and back of the fibers with different metals causes a potential difference when they are immersed in an aqueous solution. This potential difference propelled the Janus fibers in the direction perpendicular to their long axis when hydrogen peroxide was present in the solution.

Rheo-optical Analysis of Functionalized Graphene Suspensions

Wet processing of graphene sheets is a potentially interesting route for the economically viable creation of graphene-based composites. In the present work, flow dichroism and small-angle light scattering are used to investigate the dispersion of functionalized graphene sheets in a suspension and their response to shear flow. In line with expectations from scaling theory at rest, the functionalized graphene sheets are present as Brownian flat sheets, and there is no evidence of significant crumpling. More surprisingly, we find that the rate-dependent orientation of these molecularly thin sheets can be described by numerical predictions for hard spheroidal sheets, making quantitative predictions of the flow-induced orientation possible. Further comparison of the flow-induced orientation of thick gold decahedra with the thin graphene sheets shows that, except for effects of polydispersity, the flow-induced orientation is predicted well quantitatively. Adequate prediction of the effects of flow on the orientation of graphene sheets makes it possible to design wet processed graphene-based composite materials.