Papers by Author: Yun Fei Chen

Abstract: The dynamic properties of liquids in confined geometries or porous media are of both fundamental and practical importance in many physical situations, such as lubrication of micro/nanoelectromechanical systems, the flow of liquids in rocks and nanopores, and transport through porous media in filtration processes. The investigation of liquids confined at the nanoscale has been an active field for many years, but their properties remain controversial. In this work, a surface force apparatus (SFA) has been used to investigate the dynamic properties of nanoconfined octamethylcyclotetrasiloxane (OMCTS) between two mica surfaces. The dependences of normal and adhesion forces on different confinement or retraction rates were studied. The hydrodynamic effects and liquid drainage were also determined. The contribution of hydrodynamic effects to liquid drainage is limited. Our experimental results showed that normal forces are strongly changed at high loading rates, whereas adhesion forces vary slightly. The rapidly confined film behaves as a jamming liquid of enhanced viscosity for a film thickness below to a few nanometers, while the viscosity change little at slow confinement rate. These results indicate that confining rate effects play a great role in the properties of nanoconfined liquid.

Abstract: The out-of-plane thermal conductivity of silicon thin film doped with germanium is calculated by non-equilibrium molecular dynamics simulation using the Stillinger-Weber potential model. The silicon thin film is doped with germanium atoms in a random doping pattern with a doping density of 5% and 50% respectively. The effect of silicon thin film thickness on its thermal conductivity is investigated. The simulated thicknesses of silicon thin film doped with germanium range from 2.2 to 10.9 nm at an average temperature 300K. The simulation results indicate that the out-of-plane thermal conductivity of the silicon thin film doped with germanium decreases linearly with the decreasing film thickness. As for the film thickness of 9.8nm and the average temperature ranging from 250 to 1000 K, the investigation shows that the temperature dependence of the film thermal conductivity is not sensitive.

Abstract: In order to study the dynamic performance with respect to a drum type washing machine, a detailed three dimensional dynamic model considering six degrees of freedom is established in the commercial software Adams. The characteristics of model are in good consistency with the simulation results on the movement of measure point in the x-z plane and eigen modes of the rigid body system. An automatic counterbalancing model of balls with different torsion damping coefficient is designed for vibration reduction. The parameters affecting the vibration are investigated by the approach of design of experiments which can reduce design cycle significantly. Its optimization results show a good performance compared to a washing machine model without balance balls attached.

Abstract: Ionic current characterization is critical for the application of nanopores with sub 5 nm as bio medical sensors and devices. Here, we demonstrate an eccentric ionic current behavior in graphene nanopore fabricated by high resolution transmission electron microscopy (HR-TEM). A spike-like current enhancement is shown in the absence of any bio molecule or nanoparticle in the LaCl₃ and KCl solution. By tuning the hydrophobicity of the graphene surface, the spikes diminish in the current recordings acquired in graphene nanopore after 20 seconds plasma etching. We consider that the hydrophocity-induced nanobubble is present in the nanopore area, leading to the currents change as the bubbles deformation due to the voltage driven electrostatic forces on the transported ions surrounding the bubble surface.

Abstract: Nanopore and nanopore based bio-sensing technology have become into more and more interesting research area in the past ten years. In this work, micro-pore in Si-S₃N₄ chips was fabricated and characterized by Focused Ion Beam (dual Beam), and then the S₃N₄ pore was covered by Polycarbonate (PC) membrane containing 50nm nanopores and sealed by using polydimethylsiloxane (PDMS) to get hybrid micro-nanopores. The obtained chip with hybrid nanopores together with two liquid cells was integrated into an ionic current detection device for biosensing. Based on this device, λ-DNA in the electrolytic solution can be detected when it is electrophoretically driven through the hybrid nanopores, and different gestures of λ-DNA in translocation also can be discriminated.

Abstract: To understand the capture of charged polymer in salt solution, we investigated the effect of the temperature, concentration of DNA molecules and electrical field to the polynucleotide capture rate by using a single graphene nanopore. From the result of the molecular dynamics simulations, it suggested that the capture rate becomes higher with increasing temperature and that it increases exponentially with the electrical field, and that low and high concentration of the polymer do not exhibit optimal capture rate. By analyzing the simulations results we predict that the capture rate is related to the aperture and the structure of the pore.

Abstract: Nanopore and nanopore based biosensing and DNA sequencing have attracted more and more interests in the past ten years. In this paper, a simplified model is addressed to depict biomolecules passing through ultrafiltration membrane (containing nanopores). Based on this model, the passing velocity of biomolecules will not increase continuously but first increase, then decrease and stabilize with the IgG concentration increasing. Due to the physical place-holding effects and the simulation results, it can be predicted that, with biomolecules concentration increasing, the ionic current will first decrease, then increase and finnally stabilize. These predictions based on the simulation match our experimental results well.

Abstract: For studying DNA electrical double layer in electrolyte solution, the key process is to create the template for DNA processing. In this paper, we described a straightforward method for depositing DNA on a substrate. The mica was chosen as the substrate materials because of its atomically flat and negatively charged surface in solution, and mica was coated by the polylysine to absorb the DNA molecules. Using by AFM, the DNA molecules have been probed obtaining a high resolution in air and liquid, and the method is simple and easy operated. This method can be used for nano-manipulation and the quality of the DNA under physiological conditions.

Abstract: Thermal conductivities of bulk silicon are calculated by equilibrium molecular dynamics (MD) simulations. Applying common used quantum corrections to the MD results, does not bring them into better agreement with the theoretical predictions or experimental data, while the uncorrected values are closer to the theoretical predictions and experiments below 400K. By assessing the validity of quantum corrections according to theoretical calculations and MD simulations, we demonstrate that the hypothesis of equating the heat fluxes is not reliable. In addition, we explore that the rations of thermal conductivities of MD simulations and quantum calculations are approximate to 1. Then a modified quantum correction for mapping MD simulations to quantum systems is proposed.

Abstract: Contact thermal resistance between two individual silicon nanowires is investigated by nonequilibrium molecular dynamic simulations as a function of temperature, overlap, bonding strength and spacing between them. The results indicate that contact thermal resistance per unit area increases with temperature increasing. The increasing overlap leads to the increase of the contact areas, which enhances the per unit area contact thermal resistance. With a weakened interfacial van der Waals bonding strength, the contact thermal resistance per unit area increases significantly. Additionally, a method to verify the effect of the bonding strength is used by changing the interfacial spacing, and a reasonable result is observed.