Search

We demonstrate a compact (40 ${\mu}$m $\times$ 260 ${\mu}$m) spectrometer based on multimode interference aided by scattering of light from random SiO$_2$-filled hole arrays on a silicon-on-insulator platform. We characterize the performance of the spectrometer for wavelengths around 1310 nm, and report that the spectrometer can reconstruct a broadband $\sim$ 67 nm source, as well as Lorentzian probes of $\sim$ 1 nm bandwidth. This compact nanometer level resolution spectrometer can be fabricated at a low cost for lab-on-a-chip sensing and imaging applications., Comment: 11 pages, 6 figures

The average Raman signal power obtained in a modulated optical trap is dependent on the Brownian motion - therefore hydrodynamic properties of the trapped particle. Hence, in addition to the molecular properties obtained from the Raman signal, it is possible to study hydrodynamic properties (e.g. size) of the particle by analyzing the change in the average Raman power as a function of modulation frequency. Our results, based on the over-damped Langevin equation, show that several minimas exist for the Raman signal at unique modulating frequencies for a given particle size and signal acquisition time. In typical experimental conditions, such minimas can be as low as 50% of the Raman signal in an unmodulated trap.

We demonstrate an on-chip Silicon-on-Insulator (SOI) axicon etched using a low resolution (200 nm feature size, 250 nm gap) deep-ultraviolet lithographic fabrication. The axicon consists of circular gratings with seven stages of 1x2 multimode interferometers. We present a technique to apodize the gratings azimuthally by breaking up the circles into arcs which successfully increased the penetration depth in the gratings from $\approx$5 $\mu$m to $\approx$55 $\mu$m. We characterize the device's performance by coupling 1300$\pm$50 nm swept source laser in to the chip from the axicon, and measuring the out-coupled light from a grating coupler. Further, we also present the implementation of balanced homodyne detection method for the spectral characterization of the device and show that the position of the output lobe of the axicon does not change significantly with wavelength.

We investigate dilation-induced surface deformations in a Discontinuous Shear Thickening (DST) suspension to determine the relationship between dilation and stresses in DST. Video is taken at two observation points on the surface of the suspension in a rheometer while shear and normal stresses are measured. A roughened surface of the suspension is observed as particles poke through the liquid-air interface, corresponding to dilation. Dilation events are found to be intermittent and localized spatially. Shear and normal stresses also fluctuate between high- and low-stress states, and dilation is observed frequently in the high stress state. On the other hand, a complete lack of dilation is observed when the stresses remain at low values for a several seconds. Dilation is most prominent while the stresses grow from the low-stress state to the high-stress state, and the dilated region tends to span the entire surface by the end of the stress growth period. Dilation is found only at stresses and shear rates in and above the shear thickening range. These observed relations between surface dilation and stresses confirm that dilation and stresses are coupled in the high-stress state of DST., Comment: 14 pages, 9 figures

We present a technique for obtaining an effective packing fraction for discontinuous shear thickening suspensions near a critical point. It uses a measurable quantity that diverges at the critical point -- in this case the inverse of the shear rate $\dot\gamma_c^{-1}$ at the onset of discontinuous shear thickening -- as a proxy for packing fraction $\phi$. We obtain an effective packing fraction for cornstarch and water by fitting $\dot\gamma_c^{-1}(\phi)$, then invert the function to obtain $\phi_{eff}(\dot\gamma_c)$. We further include the dependence of $\dot\gamma_c^{-1}$ on the rheometer gap $d$ to obtain the function $\phi_{eff}(\dot\gamma_c,d)$. This effective packing fraction $\phi_{eff}$ has better resolution near the critical point than the raw measured packing fraction $\phi$ by as much as an order of magnitude. Furthermore, $\phi_{eff}$ normalized by the critical packing fraction $\phi_c$ can be used to compare rheology data for cornstarch and water suspensions from different lab environments with different temperature and humidity. This technique can be straightforwardly generalized to improve resolution in any system with a diverging quantity near a critical point., Comment: 8 pages, 6 figures. Submitted to Physical Review E

We experimentally characterize the impact response of concentrated suspensions of cornstarch and water. We hypothesize that the dynamically jammed region that propagates ahead of the impactor is responsible for the strong stress response to impact when it spans between solid boundaries. Using surface imaging and particle tracking at the boundary opposite the impactor, we observed that a visible structure and particle flow at the boundary occur with a delay after impact. We show the delay time is about the same time as the the strong stress response, confirming that the strong stress response results from deformation of the dynamically jammed structure once it spans between the impactor and a solid boundary. A characterization of this strong stress response is reported in a companion paper (arXiv:1407.0719). We also elaborate on the structure of the dynamically jammed region once it spans from the impactor to a solid boundary. We observed particle flow in the outer part of the dynamically jammed region at the bottom boundary, with a net transverse displacement of up to about 5\% of the impactor displacement, indicating shear at the boundary. Direct imaging of the surface of the outer part of the dynamically jammed region reveals a change in surface structure that appears the same as the result of dilation in other cornstarch suspensions. Imaging also reveals cracks, like a brittle solid. These observations suggest the dynamically jammed structure can temporarily support stress according to an effective modulus, like a soil or dense granular material, along a network of frictional contacts between the impactor and solid boundary., Comment: This was originally part of a separate paper (arXiv:1407.0719v3), before being split off as its own paper

We investigated the transient relaxation of a Discontinuous Shear Thickening suspension of cornstarch in water. We performed 2 types of relaxation experiments starting from a steady shear in a parallel plate rheometer, followed by either stopping the top plate rotation and measuring the transient torque relaxation, or removing the torque on the plate and measuring the transient tool rotation. We found that at low weight fraction $\phi_{eff}<58.8\pm0.4\%$, the suspensions exhibited a relaxation behavior consistent with a generalized Newtonian fluid. However, for larger weight fraction $58.8\% < \phi_{eff} < 61.0\%$, near the liquid-solid transition $\phi_c=61.0\pm0.7\%$, we found relaxation behaviors different from the generalized Newtonian model. The relaxation time in this range scales with the inverse of the critical shear rate at the onset of shear thickening. In this range the relaxation time was the same in both stress and rate controlled experiments, rather than the viscosity calculated from the relaxation time which is expected to be intrinsic material parameter in the generalized Newtonian model. The discrepancy between the measured relaxation times and the generalized Newtonian prediction was found to be up to $10^4$, and extrapolations diverge in the limit of $\phi_c$ as the generalized Newtonian prediction approaches 0. At the highest weight fractions, the relaxation time scales were measured to be on the order of $\sim 1$ s. The fact that this timescale is resolvable by the naked eye may be important to understanding some of the dynamic phenomenon commonly observed in these systems. We also showed that using the critical shear rate $\dot\gamma_c$ at the onset of shear thickening to characterize the effective weight fraction can more precisely characterize material properties near the critical point $\phi_c$, allowing us to resolve this transition so close to $\phi_c$., Comment: Version 3 is the final version of the paper accepted to Physical Review Fluids. Full abstract is in the file, as it is longer than the character limit on arxiv

We experimentally characterize the impact response of concentrated suspensions consisting of cornstarch and water. We observe that the suspensions support a large normal stress -- on the order of MPa -- with a delay after the impactor hits the suspension surface. We show that neither the delay nor the magnitude of the stress can yet be explained by either standard rheological models of shear thickening in terms of steady-state viscosities, or impact models based on added mass or other inertial effects. The stress increase occurs when a dynamically jammed region of the suspension in front of the impactor propagates to the opposite boundary of the container, which can support large stresses when it spans between solid boundaries. We present a constitutive relation for impact rheology to relate the force on the impactor to its displacement. This can be described in terms of an effective modulus, but only after the delay required for the dynamically jammed region to span between solid boundaries. Both the modulus and the delay are reported as a function of impact velocity, fluid height, and weight fraction. We report in a companion paper to this one on the structure of the dynamically jammed region when it spans between the impactor and the opposite boundary (arXiv:1709.01133). In a direct follow-up, we show that this constitutive model can be used to quantitatively predict, for example, the trajectory and penetration depth of the foot of a person walking or running on cornstarch and water., Comment: v3: added more stress data. We removed reflected light intensity tracking data which was irreproducible, and retract our claim of a 2nd front. v4: Visualization data was moved to a separate paper(arXiv:1709.01133). v5: some content was moved to a separate arxiv submission: "Testing constitutive relations by running and walking on cornstarch and water suspensions"