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Multi-Photon Polymerization (MPP) is a technique used to fabricate complex micro-scale 3D structures using ultra-short laser pulses. Typically, MPP is used to manufacture micron-scale components in photopolymer materials. However, the development of micron scale processes that can produce components from multiple materials within a single manufacturing step would be advantageous. This would allow the inclusion of particles that are manipulated and embedded within structures with sub-micron feature sizes. To achieve this, an MPP system was combined with an optical trapping (OT) setup in order to independently manipulate microparticles in the x, y and z planes. Particles were transported into the fabrication site using the OT and encapsulated using the MPP laser. Here it is shown that combining the OT capabilities with an additive manufacturing technique enables the production of complex multi-material artifacts. [ABSTRACT FROM AUTHOR]

Within the framework of a Boltzmann-Lorentz equation, we analyze the dynamics of a granular rotor immersed in a bath of thermalized particles in the presence of a frictional torque on the axis. In numerical simulations of the equation, we observe two scaling regimes at low and high bath temperatures. In the large friction limit, we obtain the exact solution of a model corresponding to asymptotic behavior of the Boltzmann-Lorentz equation. In the limit of large rotor mass and small friction, we derive a Fokker- Planck equation for which the exact solution is also obtained. [ABSTRACT FROM AUTHOR]

We study convective motion in vertically vibrated three-dimensional granular beds by comparing the predictions of a model based on a hydrodynamic description to Navier–Stokes order with experimental results obtained using positron emission particle tracking (PEPT). The three-dimensional conservation equations relating mass, momentum and energy are solved using the finite element (FE) method for a viscous vibrofluidized bed by using only observable system parameters such as particle number, size, mass and coefficients of restitution. The mean velocity profiles from the viscous model show reasonable agreement with the experimental results at relatively low altitudes for the range of experimental values studied, though the velocity fields at higher altitudes were systematically underestimated by the model. We confirm that the convection rolls are influenced by the sidewall coefficient of restitution and demonstrate the scaling relationships that operate, where increasing amplitude of vibration leads to a reduction in the angular velocity of the rolls. [ABSTRACT FROM PUBLISHER]

The three-dimensional conservation equations relating energy and momentum transfer in a vibrated three-dimensional granular bed have been solved numerically by the finite element method. Two closures based on granular kinetic theory were used: one, the standard Fourier law relating heat flux to temperature gradient and the other, including an additional concentration gradient term. Each prediction of the two-dimensional axisymmetric granular temperature and packing fraction fields was compared against a one-dimensional model and three-dimensional experimental results, acquired using the technique of positron emission particle tracking. Both closures resulted in solutions that were in reasonable agreement with the experimental results, but it was found that differences between the predictions of each of the closures were relatively small in comparison to the anisotropy of the experimentally determined temperature distribution. [ABSTRACT FROM AUTHOR]

A comparison of the predictions of a simple kinetic theory with experimental and numerical results for a vibrated granular bed consisting of nearly elastic particles of two sizes has been performed. The results show good agreement between the data sets for a range of numbers of each size of particle, and are particularly good for particle beds containing similar proportions of each species. The agreement suggests that such a model may be a good starting point for describing polydisperse systems of granular flows. [ABSTRACT FROM AUTHOR]

The spontaneous formation of cracks in biscuits following baking, also known as checking, is an issue that manufacturers would like to be able to predict and avoid. Unfortunately the mechanisms driving this phenomenon are not well understood. Speckle interferometry was used to study moisture-induced in-plane strain development in biscuits. This sensitive and non-contacting technique for measuring surface displacements has two major advantages over more commonly used methods; firstly, strains can be detected at a far higher sensitivity (down to 2 × 10-6) than previously accessible and secondly the method is a whole-field technique, enabling observation of the development of strain distributions during moisture migration. For biscuits exposed to step changes in humidity, initial strain rates of up to 10-5min-1 were measured, which decreased as the moisture content approached equilibrium, leading to an accumulated strain of ~10-2 after 48 h. Under these conditions, a homogeneous, uniform strain distribution was observed. The data were used to calculate the hygroscopic expansion coefficient, which was linearly related to moisture content and provides the necessary constitutive link between strain and biscuit moisture content needed to model biscuit checking. [ABSTRACT FROM AUTHOR]

The velocity autocorrelation functions of grains moving in highly fluidized three-dimensional granular beds have been determined for the first time. Positron emission particle tracking was used to determine the location of a tracer particle. The velocity auto- and cross-correlation functions were then calculated following estimation of the velocities of the grains. The cross-correlations were found to be about zero for all times and the velocity autocorrelation functions differed in the x and y directions. A numerical study was performed to model the velocity auto-correlation function in the y-direction and this analysis was found to capture many of the observed experimental features. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An analysis of the material properties of porcine corneas has been performed. A simple stress relaxation test was performed to determine the viscoelastic properties and a rheological model was built based on the Generalized Maxwell (GM) approach. A validation experiment using nano-indentation showed that an isotropic GM model was insufficient for describing the corneal material behaviour when exposed to a complex stress state. A new technique was proposed for determining the properties, using a combination of nano-indentation experiment, an isotropic and orthotropic GM model and inverse finite element method. The good agreement using this method suggests that this is a promising technique for measuring material properties in vivo and further work should focus on the reliability of the approach in practice., (© 2013 Elsevier Ltd. All rights reserved.)

The response to vibration of a granular bed, consisting of a standard cylindrical geometry but with the addition of a dissipative cylindrical inner wall, has been investigated both experimentally (using positron emission particle tracking) and numerically (using hard sphere molecular dynamics simulation). The packing fraction profiles and granular temperature distributions (in both vertical and horizontal directions) were determined as a function of height and distance from the axis. The two sets of results were in reasonable agreement. The molecular dynamics simulations were used to explore the behavior of the granular bed in the inner wall-outer wall coefficient of restitution phase space. It was observed that one could control the direction of the toroidal convection rolls by manipulating the relative dissipation at the inner and outer walls via the coefficients of restitution, and with several layers of grains it was seen that double convection rolls could also be formed, a result that was subsequently confirmed experimentally.

An investigation into the granular temperature distributions of a binary vibrofluidized granular bed has been conducted using positron emission particle tracking. By repeating each experiment with the tracer selected in turn from the two size components, the granular temperature and packing fraction distributions for each phase were determined. It was found that, for a range of size fractions, the granular temperature of the larger particles was higher than that of the smaller diameter grains, a result which was supported by a simple theoretical analysis based on the steady state energy equation.

A stochastic approach, similar to that used to describe Brownian motion, was used to model the displacement probability of grains in a three-dimensional vibrofluidized granular bed. As neither an analytical description nor measurements of the diffusion coefficients were available, the governing partial differential equation, namely, the Smoluchowski equation, was solved numerically using an iterative procedure, modifying the granular temperature profile at each step. The results of this stochastic model were compared to experimental measurements of the displacement probability density made using positron emission particle tracking. The results indicate that methods based on hard elastic systems such as the Smoluchowski equation are appropriate to granular systems, particularly over timescales greater than the mean collision time.

The motion of grains in a three-dimensional vibrofluidized granular bed has been measured using the technique of positron emission particle tracking, to provide three-dimensional packing fraction and granular temperature distributions. The mean square fluctuation velocity about the mean was calculated through analysis of the short time mean squared displacement behavior, allowing measurement of the granular temperature at packing fractions of up to eta approximately 0.15. The scaling relationship between the granular temperature, the number of layers of grains, and the base velocity was determined. Deviations between the observed scaling exponents and those predicted by recent theories are attributed to the influence of dissipative grain-sidewall collisions.

Free, buoyancy-driven convection has been observed experimentally in three-dimensional highly fluidized granular flows for the first time. Positron emission particle tracking was used to determine the position of a tracer grain in a vibrofluidized bed, from which packing fraction distributions as well as the velocity fields could be determined. The convection rolls, although small compared to the magnitude of velocity fluctuations (<5%), were consistently observed for a range of grain numbers and shaker amplitudes. Density variations are a signature of free convection and, with negative temperature gradients also present, were interpreted as the mechanism by which the convection rolls were initiated.

The analogy of vibrofluidized granular beds with a thermal gas of hard discs has been tested. Analysis of the mean squared displacement behavior of the grains allowed comparison of the measured diffusion with the predicted value at a particular combination of granular temperature and packing fraction. High speed photography, image analysis, and particle tracking software enabled accurate location of the grains. Appropriate analysis of the three mean squared displacement regimes, ballistic, diffusive, and crossover between the two extremes, allowed both the diffusion coefficient and the granular temperature to be measured at the same packing fraction. Broad agreement between Chapman-Enskog theory relating temperature to self-diffusion and observation was observed up to packing fractions of eta approximately equal to 0.7. At higher packing fractions the grains showed evidence of caging and jump diffusion, with the observed diffusion rapidly diverging from that predicted by theory. Measurement of self-diffusion coefficients and subsequent use of kinetic theory was found to be an accurate method to determine the granular temperature for intermediate packing fractions (eta=0.4-0.6), and would be particularly suitable for those situations where the time resolution of the experimental facility is insufficient to resolve the speed of the grain between collisions.