We derive a Kinetic Monte Carlo model for studying how contacts form between confined surfaces in an ideal solution. The model incorporates repulsive and attractive surface-surface forces between a periodic (2+1)-dimensional solid-on-solid (SOS) crystal surface and a confining flat surface. The repulsive interaction is derived from the theory of electric double-layers, and the attractive interactions are Van der Waals interactions between particles on the SOS surface and the confining surface. The confinement is induced by a constant external pressure normal to the surfaces which is in mechanical equilibrium with the surface-surface forces. The system is in thermal equilibrium, and particles can deposit to and dissolve from the SOS surface. The size of stable contacts formed between the surfaces in chemical equilibrium show a non-trivial dependency on the external pressure which is phenomenologically similar to the dependency of oscillatory hydration forces on the surface-surface separation. As contacts form we find classical phenomena such as Ostwald ripening, coalescence, and primary and secondary nucleation stages. We find contacts shaped as islands, bands or pits, depending solely on the contact size relative to the system size. We also find the model to behave well out of chemical equilibrium. The model is relevant for understanding processes where the force of crystallization and pressure solution are key mechanisms.
Confinement of filamentary objects is ubiquitous in numerous biological, medical, and engineering scenarios. Quantitatively determining the mechanical interaction between flexible filaments and surface confinement is particularly challenging due to the unknown contact force induced by elasticity interacting with geometric constraints. Here, we consider a simplified model of confined filamentary object: an elastic curve under surface confinement. Local force and moment balance equation incorporating the role of contact is utilized to derive the contact force exerted by surface on the confined e…
We present contact radius and curvature corrections to the nonlocal contact formulation that take into account multi-particle interactions in elastic confined granular systems. The nonlocal contact formulation removes the classical assumption of independent contacts by taking into account the interplay of deformations due to multiple contact forces acting on a single particle. The contact radius correction considers the components of these deformations that contribute to the inter-particle contact area. The curvature correction improves the description of the contacting surface profiles by inc…
The molecular structure of moving contact lines (MCLs) and the emergence of a corresponding macroscopic dissipation have made the MCL a paradigm of fluid dynamics. Through novel averaging techniques that remove capillary waves smearing we achieve an unprecedented resolution in molecular dynamics (MD) simulations and find that they match with the continuum description obtained by finite element method (FEM) down to molecular scales. This allows us to distinguish dissipation at the liquid-solid interface (Navier-slip) and at the contact line, the latter being negligible for the rather smooth sub…
Water inside a nanocapillary becomes ordered, resulting in unconventional behavior. A profound enhancement of water flow inside nanometer thin capillaries made of graphene has been observed [B. Radha et.al., Nature (London) 538, 222 (2016)]. Here we explain this enhancement as due to the large density and the extraordinary viscosity of water inside the graphene nanocapillaries. Using the Hagen-Poiseuille theory with slippage-boundary condition and incorporating disjoining pressure term in combination with results from molecular dynamics (MD) simulations, we present an analytical theory that el…
Herein, we numerically study the rheology of a two-dimensional frictional granular system confined by constant pressure under oscillatory shear. Several scaling laws for the storage and loss moduli against the scaled strain amplitude have been found. The scaling laws in plastic regime for large strain amplitude can be understood by the angular distributions of the contact force. The scaling exponents are estimated by considering the physical mechanism.
We investigate computationally the pullout of a spherical intruder initially buried at the bottom of a granular column. The intruder starts to move out of the granular bed once the pulling force reaches a critical value, leading to material failure. The failure point is found to depend on the diameter of the granular column, pointing out the importance of particle-wall interaction in determining the material response. Discrete element simulations show that prior to failure, the contact network is essentially static, with only minor rearrangements of the particles. However, the force network, w…
Recent experimental and theoretical investigations of crystal growth from solution in the vicinity of an impermeable wall have shown that: (i) growth can be maintained within the contact region when a liquid film is present between the crystal and the substrate; (ii) a cavity can form in the center of the contact region due to insufficient supply of mass through the liquid film. Here, we investigate the influence of surface kinetics on these phenomena using a thin film model. First, we determine the growth rate within the confined region in the absence of a cavity. Growth within the contact in…
Collective dynamics of densely confined active polar disks with self- and mutual alignment
Beijing Normal University（北京师范大学）；Humboldt Universität zu Berlin（柏林洪堡大学）；Research Cluster of Excellence ‘Science of Intelligence’（智能科学卓越研究中心）；Northwestern University（西北大学）；Beijing Jiaotong University（北京交通大学）；CHuepe Labs（CHuepe实验室）
The density profiles and corresponding order parameters of the hard ellipsoids confined between two hard walls and also in contact with a single hard wall are studied using the density functional theory. The Hyper-Netted Chain (HNC) approximation is used to write excess grand potential of the system with respect to the bulk value. To simplify the calculations we use restricted orientation model (ROM) for the orientation of ellipsoids to find the density profiles and order parameters. Density functional theory shows that there is a uniaxial-biaxial phase transition near a single hard wall and a…
Friction in static and sliding contact of rough surfaces is important in numerous physical phenomena. We seek to understand macroscopically observed static and sliding contact behavior as the collective response of a large number of microscopic asperities. We develop an efficient numerical framework that can be used to investigate how the macroscopic response of multiple frictional contacts depends on long-range elastic interactions, different constitutive assumptions about the deforming contacts and their local shear resistance, and surface roughness. We approximate the contact between two ro…
We couple a free solute diffusion model to a model of crystal surface growth represented by, but not limited to, a (2 + 1)-dimensional solid-on-solid (SOS) model confined by a flat surface. We use kinetic Monte Carlo (KMC) with dissolution rates based on nearest-neighbor interactions to solve the Master equation for the surface dynamics, and we use an offlattice random walk to model the Fickian diffusion of the solute particles. The two solvers are coupled directly through deposition rates of the free particles calculated using the mean first passage time (MFPT) of deposition that is found to …
In a recent paper [Phys. Rev. X 12, 031021], we reported experimental observations of ``ultrastable'' states in a shear-jammed granular system subjected to small-amplitude cyclic shear. In such states, all the particle positions and contact forces are reproduced after each shear cycle so that a strobed image of the stresses and particle positions appears static. In the present work, we report further analyses of data from those experiments to characterize both global and local responses of ultrastable states within a shear cycle, not just the strobed dynamics. We find that ultrastable states f…
Small-amplitude dynamic atomic force microscopy (dynamic-AFM) in a simple nonpolar liquid was studied through molecular dynamics simulations. We find that within linear dynamics regime, the contact stiffness and damping of the confined film exhibit the similar solvation force oscillations, and they are generally out-of-phase. For the solidified film with integer monolayer thickness, further compression of the film before layering transition leads to higher stiffness and lower damping. We find that molecular diffusion in the solidified film was nevertheless enhanced due to the mechanical excita…
We perform combined X-ray tomography and shear force measurements on a cyclically sheared granular system with highly transient behaviors, and obtain the evolution of microscopic structures and the macroscopic shear force during the shear cycle. We explain the macroscopic behaviors of the system based on microscopic processes, including the particle level structural rearrangement and frictional contact variation. Specifically, we show how contact friction can induce large structural fluctuations and cause significant shear dilatancy effect for granular materials, and we also construct an empir…
By means of computer simulations, this work addresses adhesion of a deformable spherical capsule to a micro-rough surface consisting of a periodic array of pillars. Depending on the micro-relief topography, three different adhesion regimes have been observed: 1) weak adhesion without deformation of the membrane (fakir state); 2) strong adhesion with deformation of the capsule membrane and binding to the bottom wall (nested or contacting state); 3) impalement of the capsule by micropillars. It has been found that a periodic micro-relief implies a favorable positioning of the capsule on rough su…
Colloidal dispersions are prized as model systems to understand basic properties of materials, and are central to a wide range of industries from cosmetics to foods to agrichemicals. Among the key developments in using colloids to address challenges in condensed matter is to resolve the particle coordinates in 3D, allowing a level of analysis usually only possible in computer simulation. However in amorphous materials, relating mechanical properties to microscopic structure remains problematic. This makes it rather hard to understand, for example, mechanical failure. Here we address this chall…
