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The movement of isotropic and anisotropic particles of iron and graphite within the polymer matrix was predicted and examined by the COMSOL simulation method. The interfacial adhesion of filler particles within the matrix was investigated under surface features observation. Carbonyl Iron (CI) particles, considered to be regular with a uniform size of (1–5 µm), were mixed with irregular particles of graphite (20–150 µm) with 30 V% in quantity in a silicone rubber matrix. The particle–matrix and particle–particle interactions were analyzed from the inner surface features. The drag of non-spherical particles and particle Reynolds numbers (Rep) were taken into consideration in point force models for both the Stokes (Rep ≪ 1) and Newton regime for particle shape. Newton regime is based on the aspect ratio for particles with regular and irregular shapes. The boundary area of the irregular particles holds like an anchor inside the polymer matrix for strong adhesion; however, regular particles have partial attachment due to the gravitational pull of attraction from the bottom contact points. However, uniform distribution of isotropic particles has been observed in comparison to the anisotropic particles within the polymer matrix.

While both active and granular matter have been extensively studied, here we investigate what happens when we mix the two of them, in a model system combining microparticles and cell assemblies. On a substrate covered with polystyrene or silica microparticles, we notice two regimes in the spreading of a cell aggregate: light particles are pushed by the cells and form a ring, which bonds to the substrate by adhesion forces that oppose spreading, while for heavy particles, the cell monolayer spreads above the particle bed. In both cases, cell activity is transmitted to inert beads, leading to the formation of cell-microparticle aggregates, which flicker and diffuse. We then study the formation and the spreading of hybrid aggregates of microparticles and living cells and observe phase separations and jamming transitions. Our study may have implications on processes such as cancer metastasis and development, and may guide cancer therapies based on inert particles. Interaction of active matter with geometrical and topological constraints is a topic of intense research in the recent few years due to its potential for design and control of active flow patterns. Here, the authors experimentally study the growth and expansion of cell aggregates interleaved by passive colloidal particles, showing that inert particles can reshape the collective pattern formation in cellular aggregates.

In this communication a novel implementation was outlined to simulate surface tension and adhesion by weakly compressible Smoothed Particle Hydrodynamics (SPH). Physical effects in a liquid phase of surface tension and adhesion toward a solid surface were generated by including proper particle-particle interactions within the SPH scheme. The analytical relationship describing intensities of such forces as a function of mutual distance, was derived from the literature and was given a dimensionally consistent form. Four computational approaches were compared, including as ingredients: Monaghan or Riemann solvers, for the viscous/dissipative term of momentum conservation equation; two diverse strategies to compute the overall adhesion force acting onto each liquid particle, one exploiting as sampling points “ghost” particles (already present for boundary conditions), another one based on numerical quadrature within the support of the approximating Kernel. For very preliminary simulations a single phase scenario was considered, describing in a simplified fashion a sessile droplet experiment for wettability assessment. A semicircular liquid droplet was discretized by particles, and subjected to surface tension and adhesion forces exerted by a solid, flat substrate, coincident with a boundary of the problem domain. On the basis of results so far available, Monaghan solver exhibited superior robustness; on the other hand, Riemann solver was preferable as for the smoothness of pressure field. However, severe numerical problems were met: locally, within the droplet, spurious and unphysical negative pressure were predicted by both solvers, endowed by unexpected instability of particle flow and by a global lack of robustness at varying the analysis parameters.

Active particle assemblies can exhibit a wide range of interesting dynamical phases depending on internal parameters such as density, adhesion strength or self-propulsion. Active self-rotations are rarely studied in this context, although they can be relevant for active matter systems, as we illustrate by analyzing the motion of Chlamydomonas reinhardtii algae under different experimental conditions. Inspired by this example, we simulate the dynamics of a system of interacting active disks endowed with active torques. At low packing fractions, adhesion causes the formation of small rotating clusters, resembling those observed when algae are stressed. At higher densities, the model shows a jamming to unjamming transition promoted by active torques and hindered by adhesion. Our results yield a comprehensive picture of the dynamics of active rotators, providing useful guidance to interpret experimental results in cellular systems where rotations might play a role., 18 pages, 5 figures

Analyzing surface forces for myriad geometric structures facilitates the design of properties in interacting interfacial systems. Along these lines, we demonstrate a generalized technique that can be utilized to evaluate the orientation dependence of a particle interacting with multiple finite or semi-infinite objects. Specifically, the surface element integration technique is modified to account for surface elements of a particle not directly adjacent to the object with which it is interacting; this facilitates the analysis of objects with finite shape and with arbitrary orientations. Furthermore, as a technology-relevant proof-of-concept demonstration, the influence of van der Waals (vdW) forces on the performance and reliability of microstructured systems used for the collection of trace particles is reported. The importance of the location of the particle contact with the microstructure and the independence of vdW forces generated by each microstructure is demonstrated using the developed computational approach. Thus, the methodology presented here can ultimately be utilized for a variety of interfacial forces generated by nontrivial systems with heterogeneous properties in order to provide design motifs in a low-cost, high-throughput manner.

Background Particokinetic models are important to predict the effective cellular dose, which is key to understanding the interactions of particles with biological systems. For the reliable establishment of dose–response curves in, e.g., the field of pharmacology and toxicology, mostly the In vitro Sedimentation, Diffusion and Dosimetry (ISDD) and Distorted Grid (DG) models have been employed. Here, we used high resolution scanning electron microscopy to quantify deposited numbers of particles on cellular and intercellular surfaces and compare experimental findings with results predicted by the ISDD and DG models. Results Exposure of human lung epithelial A549 cells to various concentrations of differently sized silica particles (100, 200 and 500 nm) revealed a remarkably higher dose deposited on intercellular regions compared to cellular surfaces. The ISDD and DG models correctly predicted the areal densities of particles in the intercellular space when a high adsorption (“stickiness”) to the surface was emulated. In contrast, the lower dose on cells was accurately inferred by the DG model in the case of “non-sticky” boundary conditions. Finally, the presence of cells seemed to enhance particle deposition, as aerial densities on cell-free substrates were clearly reduced. Conclusions Our results further validate the use of particokinetic models but also demonstrate their limitations, specifically, with respect to the spatial distribution of particles on heterogeneous surfaces. Consideration of surface properties with respect to adhesion and desorption should advance modelling approaches to ultimately predict the cellular dose with higher precision. Electronic supplementary material The online version of this article (10.1186/s12951-018-0426-2) contains supplementary material, which is available to authorized users.

Podosomes are submicron adhesive and mechanosensitive structures formed by macrophages, dendritic cells and osteoclasts that are capable of protruding into the extracellular environment. Built of an F-actin core surrounded by an adhesion ring, podosomes assemble in a network interconnected by acto-myosin cables. They have been shown to display spatiotemporal instability as well as protrusion force oscillations. To analyse the entire population of these unstable structures, we have designed an automated multi-particle tracking adapted to both topographical and fluorescence data. Here we describe in detail this approach and report the measurements of individual and collective characteristics of podosome ensembles, providing an integrated picture of their activity from the complementary angles of organisation, dynamics, mobility and mechanics. We believe that this will lead to a comprehensive view of podosome collective behaviour and deepen our knowledge about the significance of mechanosensing mediated by protrusive structures.

A model for the resuspension of a monolayer of particles deposited on a flat surface is developed based on a Monte Carlo simulation of the phenomenon. Particles deposited on the surface are attached to it through an adhesion force. A turbulent flow is assumed to be responsible for the resuspension of particles. The stochastic process used for particle resuspension is based on the evaluation of probabilities depending on the ratio between adhesion and aerodynamic forces and using a Metropolis function. Although simple, the present model accounts for the main features of the resuspension flux observed experimentally and by other models. The model is able to clearly show the role that the parameters of the force distributions (both adhesion and turbulent) have on the short- and long-term resuspension flux, and it captures the intrinsic stochastic nature of the resuspension process. Fil: Benito, Jesica Gisele. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Física Aplicada; Argentina Fil: Valenzuela Aracena, Karina de Los Angeles. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Física Aplicada; Argentina Fil: Uñac, Rodolfo Omar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Física Aplicada; Argentina Fil: Vidales, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Luis. Instituto de Física Aplicada; Argentina Fil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingenieria. Departamento de Fisica. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The purpose of this paper is to implement nonlinear particle interactions into subdiffusive transport, involving chemotaxis and nonlinear effects such as volume filling and adhesion. We systematically derive nonlinear subdiffusive fractional equations with chemoattractant dependent forcing. We consider the diffusion limit of the master equation and analyse the role of nonlinear tempering in the stationary case. We study the interaction between attractive forces of anomalous aggregation and chemotaxis, with repulsive forces induced by nonlinear reactions. We show that this nonlinear interaction can prevent the phenomenon of anomalous aggregation when the local particle concentration grows too high. We also show that the effect of nonlinear tempering is to suppress the intermediate subdiffusive behaviour which results in a nonlinear advection diffusion equation in the long time limit.

In this study we present a simulation model based on a Monte Carlo method to describe the resuspension of particles deposited on a flat surface due to air flow. Particles are attached to the surface through an adhesion force, and roughness effects between the particles and the surface are taken into account using a reduction factor. Two versions of the model are developed. In the first, the stochastic process used for particle resuspension is based on the evaluation of probabilities depending on the ratio between adhesion and aerodynamics forces and using a Metropolis function. In the second version, the resuspension probabilities are evaluated from a balance between the adhesion and the aerodynamics moments acting on each particle. A detailed comparison between the model results and different previous experiments is presented. Despite its simplicity, the model has a high capacity to describe the observed behaviour of the resuspended particle fraction as a function of the air velocity. The good performance of the moment balance MC model version reveals the importance of considering the rolling mechanism in the resuspension phenomena modelling. Fil: Benito, Jesica Gisele. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada ; Argentina Fil: Uñac, Rodolfo Omar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada ; Argentina Fil: Vidales, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada ; Argentina Fil: Ippolito, Irene Paula. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina

A discrete-element/boundary-element method is developed for simulation of adhesive particle transport by traveling waves on an electric curtain. The study shows that both wall adhesion and particle–particle collisions have an important influence on particle transport on electric curtains at different wave frequencies. The most significant effect of particle collisions occurs for cases with medium frequencies in which particles with large negative-charge collect in high-concentration bands and move in a synchronous surfing mode, pushing forward particles with lower charge. Cases with higher and lower frequencies exhibited hopping motion, for which adhesion determines the range of non-transported particles.

We discuss how surface roughness influences the adhesion between elastic solids. We introduce a Tabor number which depends on the length scale or magnification, and which gives information about the nature of the adhesion at different length scales. We consider two limiting cases relevant for (a) elastically hard solids with weak (or long ranged) adhesive interaction (DMT-limit) and (b) elastically soft solids with strong (or short ranged) adhesive interaction (JKR-limit). For the former cases we study the nature of the adhesion using different adhesive force laws (F ∼ u −n , n = 1.5–4, where u is the wall-wall separation). In general, adhesion may switch from DMT-like at short length scales to JKR-like at large (macroscopic) length scale. We compare the theory predictions to results of exact numerical simulations and find good agreement between theory and simulation results.

A theoretical model is presented to describe the spatiotemporal behavior of complex liquid–solid interfaces bordered by the contact line formed between liquid-like particle and solid-rough substrate. The analysis helps us to determine the soft-nanoparticle adhesion. When the interfacial temperature is above the glass transition and the substrate is rough, we find that surface forces can change by orders of magnitude depending on the temperature, contact time, and particle size varying from atoms to microns. The critical surface tension of solid substrates and the temperature-dependent structural relaxation of soft nanoparticles are shown to have significant effects on the size-dependent adhesion.

Fine excipient particles or 'fines' have been shown to improve the dispersion performance of carrier-based formulations for dry powder inhalation. Mechanistic formulation studies have focussed mainly on explaining this positive effect. Previous studies have shown that higher drug contents may cause a decrease in dispersion performance, and there is no reason why this should not be true for fines with a similar shape, size and cohesiveness as drug particles. Therefore, the effects on drug detachment of 'fine lactose fines' (FLF, X50= 1.95 mm) with a similar size and shape as micronised budesonide were studied and compared to those of 'coarse lactose fines' (CLF, X50= 3.94 mm). Furthermore, interactions with the inhalation flow rate, the drug content and the mixing order were taken into account. The observed effects of FLF are comparable to drug content effects in that the detached drug fraction was decreased at low drug content and low flow rates but increased at higher flow rates. At high drug content the effects of added FLF were negligible. In contrast, CLF resulted in higher detached drug fractions at all flow rates and drug contents. The results from this study suggest that the effects of fines may be explained by two new mechanisms in addition to those previously proposed. Firstly, fines below a certain size may increase the effectiveness of press-on forces or cause the formation of strongly coherent fine particle networks on the carrier surface containing the drug particles. Secondly, when coarse enough, fines may prevent the formation of, or disrupt such fine particle networks, possibly through a lowering of their tensile strength. It is recommended that future mechanistic studies are based on the recognition that added fines may have any effect on dispersion performance, which is determined by the formulation and dispersion conditions. © 2014 Grasmeijer et al.

In the absence of environmental cues, a migrating cell performs an isotropic random motion. Recently, the breaking of this isotropy has been observed when cells move in the presence of asymmetric adhesive patterns. However, up to now the mechanisms at work to direct cell migration in such environments remain unknown. Here, we show that a non-adhesive surface with asymmetric micro-geometry consisting of dense arrays of tilted micro-pillars can direct cell motion. Our analysis reveals that most features of cell trajectories, including the bias, can be reproduced by a simple model of active Brownian particle in a ratchet potential, which we suggest originates from a generic elastic interaction of the cell body with the environment. The observed guiding effect, independent of adhesion, is therefore robust and could be used to direct cell migration both in vitro and in vivo., Comment: To appear in PRL. Supplementary Information file available on request

We consider one-dimensional systems of self-gravitating sticky particles with random initial data and describe the process of aggregation in terms of the largest cluster size L_n at any fixed time prior to the critical time. The asymptotic behavior of L_n is also analyzed for sequences of times tending to the critical time. A phenomenon of phase transition shows up, namely, for small initial particle speeds (``cold'' gas) L_n has logarithmic order of growth while higher speeds (``warm'' gas) yield polynomial rates for L_n., Published at http://dx.doi.org/10.1214/009117904000000900 in the Annals of Probability (http://www.imstat.org/aop/) by the Institute of Mathematical Statistics (http://www.imstat.org)

When a trapped particle is subject to the tension of a massive frictional spring (the ``tether''), the escape rate increases due to a lowering of the escape barrier. However, in addition, the escape-rate prefactor is influenced by the mass and drag contributed by the spring. We solve the full Kramers escape problem for the coupled system using a technique attributed to Langer. The prefactor in the escape rate is significantly modified by the spring parameters even in the strong-damping limit. The biophysical relevance of this problem is briefly discussed.

A theoretical analysis has been made of the processes by which interparticle adhesion can occur during the compression of tablet granulations in the absence of added fluid adhesives. A reasonable explanation for the development of adhesion in these systems is based on the occurrence of either partial fusion or partial dissolution at the surfaces of the particles induced by energy added during the compression. The possibility that these phenomena do occur gains partial support from an examination of the thermodynamics and kinetics of surface processes that may take place. The Clapeyron equation cannot be applied directly to these processes, but must be modified to take into account the nonuniformity of the pressures exerted on the particles during compression. Another factor which may affect particle adhesion is the production of highly localized frictional heat and slow diffusion of this heat from the particle surfaces into their interior.