An impressive ping-pong ball cannon can be made by placing a bottle of liquid nitrogen at the bottom of a container and quickly covering it with, say, ping-pong balls. The liquid turns rapidly into a gas whose mounting pressure explodes the bottle, sending a swarm of balls upward out of the container. Surprisingly, the container also moves upward. We study the mixing of rarefied gases in a T-shape micromixer by means of fully three-dimensional Monte-Carlo direct simulations. In contrast to previous 2D-results, the characteristics of the channel walls thermal or specular have significant effect on the mixing efficiency.

For the 3D case, we characterize the mixing efficiency in dependence on temperature and mass density of the gases. Based on kinetic theory arguments, we develop a theoretical model in good agreement with the simulation results. In particular, the theoretical prediction of system size scaling agrees well with the situation. The collision of gas-borne particles with surfaces plays an important role in many processes of particle technology such as particle separation, dry dispersion of powders and particle measuring techniques.

While for coarse particles comprehensive investigations have been performed regarding sticking and bouncing behavior, in the range of nanoparticles new issues arise e. In this contribution the different interactions elastic and plastic deformation, friction, adhesion, charge transfer between single particles as well as agglomerates impacting on solid substrates are elucidated by a combination of simulations and experiments. It was found, that size-dependent material parameters can be used to describe the collision of nanoparticles with solid substrates using continuum approaches.

The effect of the impaction on the restructuring and fragmentation was investigated leading towards a dry dispersion method for nanoparticle agglomerates at ambient pressure. Clusters in systems as diverse as metal atoms, virus proteins, noble gases, and nucleons have properties that depend sensitively on the number of constituent particles.

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To this point, magic number clusters have been exclusively found with attractive interactions as present between atoms. Here we show that magic number clusters exist in a confined soft matter system with negligible interactions. Colloidal particles in an emulsion droplet spontaneously organize into a series of clusters with precisely defined shell structures. Crucially, free energy calculations demonstrate that colloidal clusters with magic numbers possess higher thermodynamic stability than those off magic numbers.

A complex kinetic pathway is responsible for the efficiency of this system in finding its minimum free energy configuration.

### Background

Targeting similar magic number states is a strategy towards unique configurations in finite self-organizing systems across the scales. Gas bubbles immersed in a liquid and flowing through a large pressure gradient undergo volumetric deformation in addition to possible deviatoric deformation. While the high density liquid phase can be assumed to be an incompressible fluid, the gas phase needs to be modeled as a compressible fluid for such bubble flow problems.

The Rayleigh—Plesset RP equation describes such a bubble undergoing volumetric deformation due to changes in pressure in the ambient incompressible fluid in the presence of capillary force at its boundary, assuming axisymmetric dynamics. We propose a compressible-incompressible coupling of Smoothed Particle Hydrodynamics SPH and validate this coupling against the RP model in two dimensions. This study complements the SPH simulations of a different class of compressible-incompressible systems where an outer compressible phase affects the dynamics of an inner incompressible phase.

For different density ratios, a sinusoidal pressure variation is applied to the ambient incompressible liquid and the response of the bubble in terms of volumetric deformation is observed and compared with the solutions of the axisymmetric RP equation. The rate of melting of a solid and the rate of deformation of the resulting melt due to capillary forces are comparable in additive manufacturing applications. This dynamic structural change of a melting solid is extremely challenging to study experimentally.

Using meshless numerical simulations we show the influence of the flow of the melt on the heat transfer and resulting phase change. We introduce an accurate and robust Incompressible Smoothed Particle Hydrodynamics ISPH method to simulate melting of solids and the ensuing fluid-solid interaction. We present validations for the heat transfer across the free surface and the melting interface evolution, separately.

We then present two applications for this coupled multiphysics simulation method — the study of rounding of an arbitrarily shaped particle during melting and the non-linear structural evolution of three spheres undergoing agglomeration. In both the studies we use realistic transport and thermal properties for the materials so as to demonstrate readiness of the method for solving engineering problems in additive manufacturing. The mechanisms underlying triboelectric charging have a stochastic nature. We investigate how this randomness affects the distributions of charges generated on granular particles during either a single or many collisions.

The charge distributions we find in our experiments are more heavy-tailed than normal distributions with an exponential decay of the probability, they are asymmetric, and exhibit charges of both signs. Moreover, we find a linear correlation between the width and mean of these distributions. We rationalize these findings with a model for triboelectric charging which combines stochastic charge separation during contact and stochastic charge recombination after separation of the surfaces.

Our results further imply that subsequent charging events are not statistically independent. Our understanding of the structural features of foams and emulsions has advanced significantly over the last 20 years. The possibility to modify systematically the interfacial properties makes these dispersions ideal systems for the exploration of soft granular materials with complex interactions.

## Molecular dynamics - Wikipedia

We present here a first systematic analysis of the structural features of such a system using a model silicone emulsion containing millimetre-sized polyethylene glycol drops PEG. Solid-like drop surfaces are obtained by polymeric cross-linking reactions at the PEG—silicone interface. Using a novel droplet-micromanipulator, we highlight the presence of elastic, adhesive and frictional interactions between two drops. We then provide for the first time a full tomographic analysis of the structural features of these emulsions. While strong analogies with frictional hard-sphere systems can be drawn, these systems display a set of unique features due to the high deformability of the drops which await systematic exploration.

Scale modelling should be a very useful strategy for the design of lunar structures. Preventing structural damages in the lunar environment is crucial and scale models are helpful to achieve this aim. The size of these models must be scaled to take into account the different gravitational levels. Since the lunar gravity acceleration is about one-sixth of the terrestrial one, it follows that the models on Earth will be very smaller than the prototype to be realized on the Moon. This strategy will represent an opportunity for engineers working on lunar structure design, provided that the errors, both computational and experimental, related to the change of scale are quantified, allowing reliable extension of the physical scale modelling results to the prototype.

In this work, a three-dimensional finite element analysis of walls retaining lunar regolith backfill is described and discussed, in order to provide preliminary results, which can guide a future experimental investigation based on physical scale-modelling.

### Misconceptions about Gyroscopic Stabilization

In particular, computational errors related to the scale effects are assessed, with respect to a virtual prototype of the lunar geotechnical structure, and compared with errors from other sources of discrepancy, like the adopted constitutive model, the variability of the geotechnical parameters and the calculation section used in the 3D analysis. The results seem to suggest the soundness of this strategy of modelling and are likely to encourage new research, both numerical and experimental, supporting the structure serviceability assessment.

Smoothed particle hydrodynamics SPH has been widely applied to flows with free surface, multi-phase flow, and systems with complex boundary geometry. Here, we consider Poiseuille flows for a wide range of Reynolds number and find that the documented instability of SPH can be avoided by using appropriate ratio of smoothing length to particle spacing in combination with a density re-initialization technique, which has not been systematically investigated in simulations of simple shear flows.

We also probe the source of the instability and point out the limitations of SPH for wall-bounded shear flows at high Reynolds number. We report evidence of a surprising systematic onset of periodic patterns in very tall piles of disks deposited randomly between rigid walls. Independently of the pile width, periodic structures are always observed in monodisperse deposits containing up to 1 0 7 disks. The probability density function of the lengths of disordered transient phases that precede the onset of periodicity displays an approximately exponential tail.

These disordered transients may become very large when the channel width grows without bound. For narrow channels, the probability density of finding periodic patterns of a given period displays a series of discrete peaks, which, however, are washed out completely when the channel width grows. Reduction in pore water pressure is a useful strategy to improve the stability of slopes. Deep draining trenches can be used for this purpose. For the realization of deep trenches, the usual conventional construction techniques are not adequate and the use of adjacent vertical panels, built by means of the methods well-established for diaphragm walls, is necessary.

However, unbonded materials i. For this scope a bonded material such as pervious concrete can be used.

## International Journal for Multiscale Computational Engineering

It must have high permeability; filtering capacity, in order to prevent internal erosion of the soil in which the trench drain is installed; and sufficient shear strength after a short curing time to avoid the instability of adjacent previously built panels. This paper reports the hydraulic characterization of two mixtures of pervious concrete carried out in the laboratory. Hydraulic conductivity was measured in saturated conditions.

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Then, the water retention functions of the mixtures were experimentally deduced by investigating different calculation options and their impact on the simulation of seepage processes through an unsaturated soil mass, in which an ideal trench is located. We investigate experimentally the impact of heterogeneity on the capillary pressure hysteresis in fluid invasion of model porous media. While enhanced heterogeneity is usually known to increase hysteresis phenomena, we find that hysteresis is greatly reduced when heterogeneities in wettability are introduced.

On the contrary, geometric heterogeneity like bidisperse particle size does not lead to such an effect. We provide a qualitative explanation of this surprising result, resting on rather general geometric arguments. Biological organisms and artificial active particles self-organize into swarms and patterns.