Anisotropic imbibition on surfaces patterned with polygonal posts

We present and interpret lattice Boltzmann simulations of thick films spreading on surfaces patterned with polygonal posts. We show that the mechanism of pinning and depinning differs with the direction of advance, and demonstrate that this leads to anisotropic spreading within a certain range of material contact angles.     

Liquid spreading on rough metal surfaces

The influence of surface roughness on the equilibrium spreading of liquids on aluminium and stainless steel surfaces with well-characterized rough machine finishes and a well-defined technique of attaining liquid drop equilibrium has been experimentally studied. The surfaces were prepared under practical conditions, i.e. without rigorous purification or attempting to eliminate anisotropy or microheterogeneities in surface-free energy. Depending on the type of roughness, i.e. spiral-grooved, radial-grooved and porous, the advancing contact angle was in approximate agreement with one of the classical contact angle/surface roughness equations. Capillary channelling along machine grooves profoundly affected the spreading and wetting behaviour and was highly dependent on the orientation and texture of roughness. Although the observed spreading was generally smooth on all surfaces it was probable that microscopic surface asperities produce small-scale non-equilibrium contact line movements and are responsible for the extensive wetting hysteresis during drop retraction.     

Dynamics of spreading of industrial oil-in-water emulsions on steel surfaces

The spreading of industrial oils, pure and emulsified in water, has been investigated on rough steel surfaces. Oils wet the surface regularly, the spreading remains circular and the contact area can be empirically correlated to timet by a power law similar to those of pure homogeneous liquids. Similar laws can also be found for oil-in-water emulsions, but in this case the spreading behaviour is quite different: first it is not circular, irregularities appear depending on the industrial oil; then retractions can occur and eventually, with some emulsions thinning of the central zone may occur after a certain time. All these effects might be due to the actual composition of these oils which are essentially esters of fatty acids dissolved in mineral oils with various added surfactants. This kind of experiment might be a very simple test to aid the choice of an appropriate rolling-oil.     

Wetting and spreading of molten aluminium against AlN surfaces

Wetting and spreading of molten aluminium against AlN substrates were investigated between 1100 and 1290°C. The contact angles decreased linearly with time under isothermal conditions between 1100 and 1200°C. The isothermal rate of spreading of molten aluminium against AlN substrates was constant between 1220 and 1290°C and the rate increased exponentially with increasing temperature. Crystals of Al4C3 nucleated and grew on the substrate surface beneath the liquid. However, the formation of Al4C3 may not be solely responsible for the changes in contact angle and spreading. It is postulated that carbon contamination from the substrate and/or experimental equipment coupled with the low oxygen partial pressure of the chamber in the presence of graphite, were primarily responsible for the observed contact angle and spreading phenomena. The activation energy for the spreading process was 448 kJ mol-1, suggesting the presence of some chemical reaction at the interface. Carbon-rich aluminium may be initiating a continuous surface reaction with the AlN substrates by reducing the native oxide layer on the substrate surface.     

Selective cell spreading, proliferation, and orientation on micropatterned gel surfaces

Hydrogel micropatterns of 10 approximately 200 microm in width were introduced during the polymerization of 2-acrylamido-2-methyl-propane sulfonic acid sodium salt (NaAMPS) on the surface of polyacrylamide (PAAm) gel. Behaviors of endothelial cells on the micropatterned PNaAMPS/PAAm gel surfaces were studied. Cells selectively proliferate on micropatterned PNaAMPS surface but not on PAAm surface, which requires no modification with any cell adhesive proteins or peptides. We found that decrease of the width of the micropatterns could increase the degree of anisotropic spreading of cells and the degree of cell orientation. These results demonstrated that the topographical micropatterns of hydrogel could control cell behaviors.     

Molecular dynamics study of nanoscale structure formation in droplet spreading on solid surfaces

Spreading of nanosized droplets was simulated on the basis of the isothermal molecular dynamics. A conclusion is made that the nanoscale droplet evolution has many common features with the macroscopic spreading. At the same time, some structure formation processes are revealed specific for the nanosized region. Observed effects include: the pyramid-like structure formation, the orientational ordering and the parquet cluster structure formation in nanodroplets composed of rod-like molecules, the replication of the structured (heterogeneous) surfaces represented by high- and low-energy segments such as striped substrates and surfaces with quadratic high- and low-energy inclusions.     

Improved adherence and spreading of Saos-2 cells on polypropylene surfaces achieved by surface texturing and carbon nitride coating

The adhesion and contact guidance of human primary osteogenic sarcoma cells (Saos-2) were characterized on smooth, microstructured (MST) and micro- and nano-structured (MNST) polypropylene (PP) and on the same samples with a silicon-doped carbon nitride (C₃N₄-Si) coating. Injection molding was used to pattern the PP surfaces and the coating was obtained by using ultra-short pulsed laser deposition (USPLD). Surfaces were characterized using atomic force microscopy and surface energy components were calculated according to the Owens-Wendt model. The results showed C₃N₄-Si coated surfaces to be significantly more hydrophilic than uncoated ones. In addition, there were 86% more cells in the smooth C₃N₄-Si coated PP compared to smooth uncoated PP and 551%/476% more cells with MST/MNST C₃N₄-Si coated PP than could be obtained with MST/MNST uncoated PP. Thus the adhesion, spreading and contact guidance of osteoblast-like cells was effectively improved by combining surface texturing and deposition of osteocompatible C₃N₄-Si coating.     

Controlled release of chol-TEG-DNA from Nano- and micropatterned SU-8 surfaces by a spreading lipid film

We report the controlled release of immobilized cholesteryl-tetraethyleneglycol-DNA (chol-DNA) from micropatterned SU-8 surfaces by a spreading lipid film. The release of chol-DNA is rapid and on the order of the spreading rate of the lipid film beta = 1-3 microm2/s ( approximately 10(5) molecules of DNA per second). The lipid film serves as a poor solvent for the DNA adduct, which upon contact redistributes into the aqueous phase. Thus, the release of DNA is accompanied by a change in surface hydrophobicity. The method can be used for creating arbitrary concentration profiles of DNA in solution over time or to dynamically change surface properties on demand in, for example, micro- and nanofluidic devices. Examples of DNA release from spiral, comb, meander, and triangular as well as from nanoscale SU-8 lanes are shown.     

Stress computations on perforated polygonal domains

A high order accurate and fast algorithm is constructed for 2D stress problems on multiply connected finite domains. The algorithm is based on a Fredholm integral equation of the second kind with non-singular operators. The unknown quantity is the limit of an analytic function. On polygonal domains there is a trade-off between stability and rate of convergence. A moderate amount of precomputation in higher precision arithmetic increases the stability in difficult situations. Results for a loaded single edge notched specimen perforated with 1170 holes are presented. The general usefulness of integral equation methods is discussed.     

Investigation of the spreading and adhesion of human osteosarcoma cells on smooth and micro-grooved polydimethylsiloxane surfaces

The paper presents recent results of the study of spreading and adhesion of human osteosarcoma cells on soft polydimethylsiloxane (PDMS) materials. Cell/surface interactions are studied on smooth and micro-grooved PDMS surfaces. The viscoelastic spreading behavior of the human osteosarcoma cells (during the spreading stages on smooth PDMS surfaces) was studied using wetting theory. The HOS cell spreading behavior was also investigated in the micro-grooved PDMS surfaces in an effort to study the contact guidance formation process. The results show that the initial stages of HOS cell spreading can be modeled as a complete wetting process. The results also show that the cell cytoplasm contributes more to the spreading process than the nucleus. In the case of cell spreading on micro-grooved surfaces, the cell tractions resulted in significant deformation of the microgrooves. The tractions were also calculated, and found to be in good agreement with the results from other studies. The results suggest that the cell spreading-induced soft substrate deformation needs to be considered in the design of implantable bioMEMS structures.     

Peptide immobilization on polyethylene terephthalate surfaces to study specific endothelial cell adhesion, spreading and migration

To control specific endothelial cell (EC) functions, cell adhesive RGDS, EC specific REDV and YIGSR peptides, and angiogenic SVVYGLR sequences were covalently immobilized onto polyethylene terephthalate (PET) surfaces for the purpose of cell culture. X-ray photoelectron spectroscopy, atomic force microscopy, fluorescence microscopy and contact angle measurement were employed for characterization of surface modifications. The peptide density on PET surfaces was evaluated by fluorescence microscopy. The surfaces immobilized with peptides were exposed to human umbilical vein endothelial cells to study their specific effects onto EC functions. The results showed that the surface functionalized by these peptides enhanced the EC adhesion, spreading and migration as compared with native PET surfaces. Specifically, the RGDS peptides induced more cell adhesion than other peptides. The YIGSR and SVVYGLR sequences induced more cell spreading and cell migration, represented by intense focal adhesion at the leading edges of cell spreading and migration. The bi-functionalization of RGDS and SVVYGLR peptides (MIX) combined the advantages of both peptides and induced significant EC adhesion, spreading and migration. Our study indicates that the surface functionalization by peptides specific for ECs, especially the combination of RGDS with SVVYGLR or YIGSR peptides, has potential applications in promoting endothelialization of vascular prostheses and for construction of vascularized tissues in tissue engineering.     

Imbibition of Liquid Helium in Aerogels

We report optical measurements of the imbibition of liquid helium in a sample of silica aerogel with 90 % porosity. Both direct imaging and light scattering experiments were performed to determine the dynamics and the properties of the liquid-gas interface in both the normal and superfluid phases of liquid helium. In the normal phase, a classical Lucas Washburn behavior is observed for the rise of the imbibition front while the behavior in the superfluid phase is markedly different, as the fluid invades the sample from all sides with a constant speed. In both phases, the interface is rough, leading to light scattering. In addition, condensation ahead of the imbibition front is observed at low temperature in the superfluid phase.     

Numerical solutions for polygonal cracks

An effective numerical scheme capable to deal with polygonal and branching cracks in a plane is proposed. It is suggested to decompose the general singular integral equation, SIE, for curvilinear cracks into a set of SIEs for straight cuts coinciding with straight crack segments. Solutions of SIEs are sought as bounded for all internal ends of the cuts and unbounded for the left end of the left cut and the right end of the right cut. The Gauss-Chebyshev quadrature is applied to each SIE that eventually leads to an over-determined system of linear algebraic equations followed by the application of the least squares method to solve this system. Stress intensity factors are calculated for some crack configurations. This scheme provides satisfactory accuracy although no correct asymptotic behaviour of the solution at internal ends is taken into consideration. The results are verified against the known results for branching cracks.     

Manipulation of self-assembled structures by shape-designed polygonal colloids in 2D

Manipulating self-assembled structures through shape-control of constitute particles is a fascinating yet quite challenging route to make new functional materials that can be used in a variety of applications. Toward this goal, the physics underlying the relation between the shape of constitute building blocks and their self-assembled structures (shape-structure relation) is the key and need to be better understood first. With the advances in particle fabrication techniques, our library of available anisotropic building blocks has expanded enormously, which opens up new opportunities for studying the shape-structure relation. There have been extensive studies performed to explore the self-assembly of anisotropic building blocks and tremendous progress has been made. In this mini-review, we will report recent progress on the self-assembly of non-spherical colloids both in experiments and in simulations. We focus on the self-assembly of polygonal platelets with a variety of shapes in two dimensions including regular polygonal shapes and a specific type of shape, kite-shape. Associated models that are helpful to understand the shape-structure relation are also summarized. We conclude this review with a brief discussion of current challenges in the field.     

Hyperelastic analysis based on a polygonal finite element method

In this contribution, we present a novel polygonal finite element method applied to hyperelastic analysis. For generating polygonal meshes in a bounded period of time, we use the adaptive Delaunay tessellation (ADT) proposed by Constantinu et al. [1 A. Constantinu, P. Steinmann, T. Bobach, G. Farin, and G. Umlauf, The adaptive Delaunay tessellation: A neighborhood covering meshing technique., Comput. Mech., vol. 42, no. 5, pp. 655–669, 2008.[Crossref], [Web of Science ®] , [Google Scholar]]. ADT is an unstructured hybrid tessellation of a scattered point set that minimally covers the proximal space around each point. In this work, we have extended the ADT to nonconvex domains using concepts from constrained Delaunay triangulation (CDT). The proposed method is thus based on a constrained adaptive Delaunay tessellation (CADT) for the discretization of domains into polygonal regions. We involve the metric coordinate (Malsch) method for obtaining the interpolation over convex and nonconvex domains. For the numerical integration of the Galerkin weak form, we resort to classical Gaussian quadrature based on triangles. Numerical examples of two-dimensional hyperelasticity are considered to demonstrate the advantages of the polygonal finite element method.     

Scattering by random particles on optical surfaces

An analytical technique has been developed for calculating the angle-resolved light scattering by contaminant particles on smooth opaque surfaces. The analytical method was tested by a comparison of measured and calculated bidirectional reflectance distribution function (BRDF) values for contaminated surfaces. BRDF values were calculated from particle sizes, shapes, and areal densities obtained from scanning electron microscope images of the contaminated surfaces by a Princeton gamma-tech image analyzer. Measured and calculated BRDF values agreed to within the uncertainty associated with the particle characterization process for most scattering angles.     

转镜-振镜扫描的非线性及非对称性研究

根据几何光学原理,推导出转镜-振镜扫描系统物镜前振镜上光斑移动速度的解析表达式。 系统本身的结构特点是产生扫描非线性和非对称性的根源,并会引起图像畸变。对设计的40面,半径40mm,转速为23437.5rmin-1的转镜-振镜系统的数值计算结果表明,行扫描中最大速度差约为6m.s-1,时间对称中心从32祍改变到约30.7477祍。此结果可作图像畸变修正的参考。