Lucy T. Zhang

Nanoscale Wetting on Groove-Patterned Surfaces

In this paper, nanoscale wetting on groove-patterned surfaces is thoroughly studied using molecular dynamics simulations. The results are compared with Wenzels and Cassies predictions to determine whether these continuum theories are still valid at the nanoscale for both hydrophobic and hydrophilic types of surfaces when the droplet size is comparable to the groove size. A system with a liquid mercury droplet and grooved copper substrate is simulated. The wetting properties are determined by measuring contact angles of the liquid droplet at equilibrium states. Correlations are established between the contact angle, roughness factor r, and surface fraction f. The results show that, for hydrophobic surfaces, the contact angle as a function of roughness factor and surface fraction on nanogrooved surfaces obeys the predictions from Wenzels theory for wetted contacts and Cassies theory for composite contacts. However, slight deviations occur in composite contacts when a small amount of liquid penetration is observed. The contact angle of this partial wetting cannot be accurately predicted using either Cassies or Wenzels theories. For hydrophilic surfaces, only wetted contacts are observed. In most cases, the resulting contact angles are found to be higher than Wenzels predictions. At the nanoscale, high surface edge density plays an important role, which results in contact line pinning near plateau edges. For both hydrophobic and hydrophilic surfaces, substantial amount of anistropic spreading is found in the direction that is parallel to the grooves, especially at wetted or partially wetted contacts.

On computational issues of immersed finite element methods

The objective of this paper is to provide a review of recent finite element formulations for immersed methods. In these finite element formulations, independent Lagrangian solid meshes are introduced to move on top of a background Eulerian fluid mesh. This key feature allows the handling, without excessive fluid mesh adaptation, multiple deformable solids immersed in viscous fluid. In particular, pros and cons of both explicit and implicit approaches are illustrated along with subtle differences between incompressible and slightly compressible models.

Thermostats and thermostat strategies for molecular dynamics simulations of nanofluidics

The thermostats in molecular dynamics (MD) simulations of highly confined channel flow may have significant influences on the fidelity of transport phenomena. In this study, we exploit non-equilibrium MD simulations to generate Couette flows with different combinations of thermostat algorithms and strategies. We provide a comprehensive analysis on the effectiveness of three thermostat algorithms Nosé-Hoover chain (NHC), Langevin (LGV) and dissipative particle dynamics (DPD) when applied in three thermostat strategies, thermostating either walls (TW) or fluid (TF), and thermostating both the wall and fluid (TWTF). Our results of thermal and mechanical properties show that the TW strategy more closely resembles experimental conditions. The TF and TWTF systems also produce considerably similar behaviors in weakly sheared systems, but deviate the dynamics in strongly sheared systems due to the isothermal condition. The LGV and DPD thermostats used in the TF and TWTF systems provide vital ways to yield correct dynamics in coarse-grained systems by tuning the fluid transport coefficients. Using conventional NHC thermostat to thermostat fluid only produces correct thermal behaviors in weakly sheared systems, and breaks down due to significant thermal inhomogeneity in strongly sheared systems.

Cell and nanoparticle transport in tumour microvasculature: the role of size, shape and surface functionality of nanoparticles

Through nanomedicine, game-changing methods are emerging to deliver drug molecules directly to diseased areas. One of the most promising of these is the targeted delivery of drugs and imaging agents via drug carrier-based platforms. Such drug delivery systems can now be synthesized from a wide range of different materials, made in a number of different shapes, and coated with an array of different organic molecules, including ligands. If optimized, these systems can enhance the efficacy and specificity of delivery compared with those of non-targeted systems. Emerging integrated multiscale experiments, models and simulations have opened the door for endless medical applications. Current bottlenecks in design of the drug-carrying particles are the lack of knowledge about the dispersion of these particles in the microvasculature and of their subsequent internalization by diseased cells (Bao et al. 2014 J. R. Soc. Interface 11, 20140301 (doi:10.1098/rsif.2014.0301)). We describe multiscale modelling techniques that study how drug carriers disperse within the microvasculature. The immersed molecular finite-element method is adopted to simulate whole blood including blood plasma, red blood cells and nanoparticles. With a novel dissipative particle dynamics method, the beginning stages of receptor-driven endocytosis of nanoparticles can be understood in detail. Using this multiscale modelling method, we elucidate how the size, shape and surface functionality of nanoparticles will affect their dispersion in the microvasculature and subsequent internalization by targeted cells.

Characterizing left atrial appendage functions in sinus rhythm and atrial fibrillation using computational models

Clinical studies show that the left atrial appendage, a blind-ended structure that is attached to the left atrium, may be the cause of 90% of atrial thrombi in atrial fibrillation (abnormal heart rhythm), and it is much reduced in sinus (normal) rhythm. In this paper, the effects of blood flows in left atrium and left atrial appendage are studied to help characterize the atrial appendage functions in sinus rhythm and atrial fibrillation using mathematical models. Our results show that the left atrial appendage is not functional in sinus rhythm because the atrial transmitral velocities remained almost identical for atria with and without appendage, which agrees with the current clinical observations. However, in atrial fibrillation, a proper atrial contraction is absent, which causes the second emptying velocity (A-wave) to be missing in both transmitral velocity and appendage filling/emptying velocity. Without the proper emptying of the blood, vortices generated in the chamber remain high strengths and with longer durations. They induce ineffective emptying of the blood in the atrium and appendage, which then lead to blood stagnation and subsequent thrombus formation.

Numerical studies on fluid–structure interactions of stent deployment and stented arteries

This study is to investigate the mechanical behaviors of angioplasty stents during and after implantation and blood flow in the stented artery using the immersed finite element method. In this study, the mechanical behaviors such as the expansion mechanism, stress distribution on the stent during the implantation, and stent responses toward various vascular conditions are analyzed. Furthermore, pulsatile blood flow in stented artery is also examined to identify potential regions where the blood clots form and potentially induce reactions leading to thrombus formation. We found that the wall shear stress and the residence time are the highest at the entrance and the exit of the stent while they decrease significantly in between stent struts. The results suggested that platelets/particles are most likely to be activated near the entrance; they form aggregates in between struts where the shear stress is very low and eventually reside at the end of the stent where the resident time is the highest. Our numerical observations agree quite well with in vitro experimental studies. This analysis will assist in the development of novel stent designs and stent deployment protocols to minimize vascular injury during stenting and reduce restenosis.

Modeling of Soft Tissues Interacting with Fluid (Blood or Air) Using the Immersed Finite Element Method.

This paper presents some biomedical applications that involve fluid-structure interactions which are simulated using the Immersed Finite Element Method (IFEM). Here, we first review the original and enhanced IFEM methods that are suitable to model incompressible or compressible fluid that can have densities that are significantly lower than the solid, such as air. Then, three biomedical applications are studied using the IFEM. Each of the applications may require a specific set of IFEM formulation for its respective numerical stability and accuracy due to the disparities between the fluid and the solid. We show that these biomedical applications require a fully-coupled and stable numerical technique in order to produce meaningful results.

Survey of Multi-scale Meshfree Particle Methods

A multiscale meshfree particle method is developed, which includes recent advances in SPH and other meshfree research efforts. Key features will include linear consistency, stability, and both local and global conservation properties. In addition, through the incorporation of Reproducing Kernal Particle Method (RKPM), standard moving least squares (MLS) enhancement and wavelet techniques, the method have the flexibility of resolving multiple scales in the solution of complex, multiple physics processes. We present the application of this approach in the following areas: 1) simulations on propagation of dynamic fracture and shear band; 2) impact and penetration; 3) fluid dynamics and 4) nano-mechanics.

Connectivity-free front tracking method for multiphase flows with free surfaces

In this study, a connectivity-free front tracking method is developed to simulate multiphase flows with free surfaces. This method is based on the point-set method which does not require any connectivities between interfacial points to represent the interface. The main advantage of the connectivity-free approach is the easiness in re-constructing the interface when large topology change occurs. It requires an indicator field to be constructed first based on the existing interface and the surface curvature and normal are then computed using the indicator field. Here, we adopt the reproducing kernel particle method (RKPM) interpolation function that provides the ability to deal with free-surface flows and the flexibility of using non-uniform meshes when local fine resolution is needed. A points regeneration scheme is developed to construct smooth interfaces and to automatically handle topology changes. The mass conservation is verified by performing a single vortex advection test. Several 2-D and 3-D numerical tests including an oscillating droplet, dam-breaking, two droplet impacting and multi-bubble merging are presented to show the accuracy and the robustness of the method.

Toward Generating Low-Friction Nanoengineered Surfaces with Liquid–Vapor Interfaces

Using molecular dynamics (MD), we investigate the importance of liquid-vapor interface topography in designing low-friction nanoengineered superhydrophobic surfaces. Shear flow is simulated on patterned surfaces. The relationship between the effective slip length and bubble meniscus curvature is attained by generating entrapped bubbles with large protrusion angles on patterned surfaces with nanoholes. We show that protruding bubbles can induce significant friction, which hinders the slip characteristics produced on liquid-vapor interfaces. By comparing surfaces with nanoholes and nanopillars, we also demonstrate that the continuity of the liquid-vapor interface can greatly influence slip. Our MD results yield an asymptotic behavior of slip length with varying gas fractions, which are found to be consistent with observations from simulations and analytical models produced in continuum studies.