Implementation of a piezoresistive MEMS cantilever for nanoscale force measurement in micro/nano robotic applications

The nanoscale sensing and manipulation have become a challenging issue in micro/nanorobotic applications. In particular, a feedback sensor-based manipulation is necessary for realizing an efficient and reliable handling of particles under uncertain environment in a micro/ nano scale. This paper presents a piezoresistive MEMS cantilever for nanoscale force measurement in microrobotics. A piezoresistive MEMS cantilever enables sensing of gripping and contact forces in nanonewton resolution by measuring changes in the stress-induced electrical resistances. The calibration of a piezoresistive MEMS cantilever is experimentally carried out. In addition, as part of the work on nanomanipulation with a piezoresistive MEMS cantilever, the analysis on the interaction forces between a tip and a material, and the associated manipulation strategies are investigated. Experiments and simulations show that a piezoresistive MEMS cantilever integrated into a microrobotic system can be effectively used in nanoscale force measurements and a sensor-based manipulation.     

Spinning wheel powder feeding device — fundamentals and applications

A spinning wheel powder feeding system has been developed as a conveying mechanism to feed fine particle aggregates on a laboratory scale. An example of a use of this conveying mechanism is with a transport tube reactor, since the reactor only provides a few seconds residence time to react the powder. Methods to shear the powder mechanically, as opposed to using a high gas velocity, are developed as to not reduce the available residence time in the reactor. The objective is to feed a powder at the smallest particle aggregate size possible rather than a large particle aggregate size generated by an upstream feeding device, and to achieve such dispersion using minimized gas flow. Statistical results show that the spinning wheel alone is able to reduce the mean aggregate size of the Particle Size Distribution (PSD) and when a minimal amount of gas is added to the system the PSD is reduced further. In addition, a fundamental model employing a discrete particle aggregate breakage equation combined with a Monte Carlo method has shown that the spinning wheel feeding system is able to consistently reduce particle aggregate size.     

Fast Transport of Colloidal Particles through Quartz-Packed Columns

Abstract

The migration of polystyrene latex particles (diameter = 310, 170 and 40 nm) through columns packed with 30 um quartz powder was experimentally studied at low eluant velocity at pH 6.0 + 0.1. The average velocity of polystyrene latex particle was found to be larger than that of HTO. The ratio (separation factor) of the average velocity of polystyrene latex particle to that of HTO increased with increasing the size of polystyrene latex particle and with decreasing the concentration of sodium chloride added under the experimental conditions. This reveals that the colloid migration process in the geological media is similar to that occurring in hydrodynamic chromatography. A model proposed for hydrodynamic chromatography was used to calculate the separation factor in which the potentials from the double layer force and from the van der Waals force were taken into account. The separation factor, calculated with the model by using the measured surface potentials of polystyrene latex particle and quartz powder and by using Hamaker constant evaluated from the literature, was found to agree well with the experimental result.     

A star-function based density functional study of the adsorption of Lennard-Jones fluid near its supercritical states

The goal of this paper is to show that the wetting behavior of simple fluids on a repulsive solid surface - especially the “drying” phenomenon - is closely related to the proximity of the supercritical state of the bulk fluids to their vapor-liquid coexistence region. We propose here a new DFT (the star-function based density functional theory, s-DFT) that is based on the functional Taylor expansion of the intrinsic free energy F[ρ] and the singlet direct correlation to arrive at closed-form expressions for both quantities without truncations or approximations. The two formulas are mutually consistent because of the introduction of a star function Sw that has been shown to be the functional primitive of the bridge function Bw, i.e. (L.L. Lee, J. Chem. Phys. 97 (1992) 8606 [34]). The new formulation is applied to the Lennard-Jones molecules adsorbed on a planar hard wall (LJ/HW). We carried out new Monte Carlo simulations for this system. Since the s-DFT uses a bridge function Bw, we demonstrate (i) the existence of a set of data (inverted from the MC information) that can perform as the bridge function and reproduce accurately the density profiles ; (ii) this set of data can be “fitted” by a function-form with acronym ZSEP; finally (iii) ZSEP expresses the bridge function Bw in terms of a new renormalized basis function γ_H, i.e. Bw(γH). The existence of a bridge function dispels some of the misconceptions that the bridge-function based formulations did not describe the “drying” behavior. We also show that for the high density case ZSEP equation can qualify as a “closure relation”, but seems to deteriorate for the two low-density supercritical states that are close to the bulk saturated liquid phase boundaries.     

The Laboratory Founded by Van der Waals

During the past century, the Van der Waals Laboratory at the University of Amsterdam has been the principal provider of reliable fluid property data over large ranges of pressure and temperature. This paper describes the history of the laboratory, starting in 1898 when funding for it was obtained. In the early period, under Van der Waals and Kohnstamm, the high-pressure direction was chosen, and the first PVT and phase equilibria data were published. The main focus of this paper is the Michels period, from 1921 to 1960. In this period, the laboratory acquired its own building and assumed a unique position in the world because of its highly accurate thermodynamic, transport, and other property measurements in fluids at high pressures. In the 1950s, a second laboratory was built by Michels at the University of Maryland, per request of the U.S. Navy. Under Trappeniers, 1961–1987, the laboratory incorporated new techniques, such as NMR, undertook a major expansion of the pressure range, and extended its interest to phase transitions in molecular solids. The position of the Van der Waals Laboratory in the world of high-pressure science is highlighted.     

Capillary and van der Waals force between microparticles with different sizes in humid air

It is well known that surface effect forces, such as van der Waals force and capillary force, are the major contributions to adhesion when microsized particles are in contact in humid environment. But it is very complex to calculate the adhesion force between two smooth unequal particles. In conventional approaches, the effective particle radius approximation and the constant half-filling angle assumptions are often used for computing the van der Waals forces between two microparticles. However, the approximation and the assumption are actually difficult to accurately model the forces between unequal particle sizes when the surfaces are with different properties. In this paper, we present a theoretical study of the van der Waals force and capillary force between two microparticles with different radii and the surface properties linked by a liquid bridge. The proposed model provides the adhesion force predictions in good agreement with the previous formula and existing experiment data. Considering the solid particles are partially wetted by the liquid bridge, the van der Waals force is calculated by divided the particle surface into a wetted part and a dry portion in our stimulation. Since the wetted surface portion of the particle is determined by the half-filling angle, the relationship between two half-filling angles of the unequal size particles is developed from the geometrical consideration, which is relate to the size ratio of the particles, the contact angle, and the separation distance. Then, the van der Waals force is determined using the surface element integration. Moreover, the influences of humidity, particles size, contact angle, and separation distance toward the adhesion forces are discussed using the proposed method. Simulations indicate that a higher relative humidity leads to bigger liquid bridges, suggesting a higher capillary force, but at the same time, the van der Waals force decreases due to the decrease in surfaces energy. As for the influence of contact angle, results show that a higher contact angle, that is, a more hydrophobic surface, reduces the capillary force but increases the van der Waals force (absolute value). The simulations also show that the both the capillary force and the van der Waals force (absolute value) increase as the particle size increases. When the particles are separated from each other, the capillary force and van der Waals force decreases gradually. These results are helpful to understand and utilize the adhesion interaction between particles with unequal sizes at the ambient condition.     

二硫化钼-石墨烯垂直异质结的制备与表征

以采用化学气相沉积法(CVD)生长的单层石墨烯为导电电极、四硫代钼酸铵水溶液为电解质,通过电化学沉积法合成了二硫化钼/石墨烯(MoS2/graphene)垂直异质结。将合成的MoS2/graphene垂直异质结通过CVD在氢气(H2)和氩气(Ar)环境下进行退火处理。利用拉曼光谱、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、原子力显微镜(AFM)系统地分析了样品的物质成分、表面形貌和厚度等。这种简单、环保、低成本的制备大面积MoS2/graphene垂直异质结的方法具有普遍适用性,为其他垂直异质结的制备开辟了新途径。     

Monte Carlo Simulations of Binary Lennard–Jones Mixtures: A Test of the van der Waals One-Fluid Model

Monte Carlo simulations in the canonical ensemble have been performed in the liquid and supercritical regions of a binary Lennard–Jones mixture with differences in size parameters of 6.4% and energy parameters of 37%. The results are compared with a recent fundamental equation of state employing the van der Waals one-fluid model and new simulation data at the corresponding state conditions of the pure Lennard–Jones fluid. The van der Waals one-fluid model describes the mixture properties well at high densities, while at low densities the predicted internal energies and isochoric heat capacities are too low.     

A size-dependent model for instability analysis of paddle-type and double-sided NEMS measurement sensors in the presence of centrifugal force

Paddle-type and double-sided nanostructures have much potential for measuring the angular speed in rotary systems and aerospace applications. While most investigations in the literature have concentrated on the electromechanical performance of conventional beam-type nanostructures, few researchers have addressed the performance of these systems. The pull-in instability of the cantilever paddle-type and double-sided sensors in the presence of the centrifugal force have been investigated. The nonlinear governing equations are solved and the obtained results are compared with the numerical solution. The influences of the van der Waals force, geometric parameters, angular speed, and size phenomenon on the instability performance have been demonstrated.