Ya-Pu Zhao

Measurement of the Rate of Water Translocation through Carbon Nanotubes

We present an approach for measuring the water flow rate through individual ultralong carbon nanotubes (CNTs) using field effect transistors array defined on individual tubes. Our work exhibits a rate enhancement of 882-51 and a slip length of 53-8 nm for CNTs with diameters of 0.81-1.59 nm. We also found that the enhancement factor does not increase monotonically with shrinking tube diameter and there exists a discontinuous region around 0.98-1.10 nm. We believe that these single-tube level results would help understand the intrinsic nanofluidics of water in CNTs.

Size effect on the coalescence-induced self-propelled droplet

An analysis based on the energy conservation is presented for the self-propelled droplet during coalescence of two droplets of the same size over a superhydrophobic rough surface. The self-propelled behavior occurs only for the coalescence of droplets with a certain range of radius. An analytical relation is established among the coalescence-induced velocity, surface energy, viscous dissipation, and droplet size if gravity is negligible. The coalescence-induced velocity increases with increasing droplet size to a maximum and then decreases with the size, which is in good accord with the experimental observation reported in the literature

Hydroelectric Voltage Generation Based On Water-Filled Single-Walled Carbon Nanotubes

A DFT/MD mutual iterative method was employed to give insights into the mechanism of voltage generation based on water-filled single-walled carbon nanotubes (SWCNTs). Our calculations showed that a constant voltage difference of several mV would generate between the two ends of a carbon nanotube, due to interactions between the water dipole chains and charge carriers in the tube. Our work validates this structure of a water-filled SWCNT as a promising candidate for a synthetic nanoscale power cell, as well as a practical nanopower harvesting device at the atomic level.

Modelling analysis of surface stress on a rectangular cantilever beam

Three models of surface stress on a rectangular cantilever beam are presented. The surface stress is modelled as a corresponding concentrated moment at the beam free end, a corresponding concentrated moment plus a corresponding concentrated axial load at the beam free end, and a corresponding uniformly distributed axial stress plus bending moment per unit length along the beam span, respectively. The results of the three models are compared under three different loading scenarios. We also present an analysis of the error source, when using Stoney’s formula to predict the surface stress, by comparing the kinematic and loading assumptions of the three models. The surface stress effects on structure deflection are usually modelled as bending moments applied at structure free edge(s)/end(s). Modelling the surface stress effect along the beam neutral axis is presented and compared with modelling its effect at free edge(s)/end(s). The stiffening effect of tensile surface stress is also studied.

Deformation of PDMS membrane and microcantilever by a water droplet: Comparison between Mooney–Rivlin and linear elastic constitutive models

In this paper, we studied the role of vertical component of surface tension of a water droplet on the deformation of membranes and microcantilevers (MCLs) widely used in lab-on-a-chip and micro- and nano-electromechanical system (MEMS/NEMS). Firstly, a membrane made of a rubber-like material, poly(dimethylsiloxane) (PDMS), was considered. The deformation was investigated using the Mooney-Rivlin (MR) model and the linear elastic constitutive relation, respectively. By comparison between the numerical solutions with two different models, we found that the simple linear elastic model is accurate enough to describe such kind of problem, which would be quite convenient for engineering applications. Furthermore, based on small-deflection beam theory, the effect of a liquid droplet on the deflection of a MCL was also studied. The free-end deflection of the MCL was investigated by considering different cases like a cylindrical droplet, a spherical droplet centered on the MCL and a spherical droplet arbitrarily positioned on the MCL. Numerical simulations demonstrated that the deflection might not be neglected, and showed good agreement with our theoretical analyses.

Hierarchical self-assembly of achiral amino acid derivatives into dendritic chiral nanotwists.

The organogel formation and self-assembly of a glycine-based achiral molecule were investigated. It has been found that the compound could gel organic solvents either at a lower temperature with lower concentration or at room temperature with higher concentration, which showed different self-assembled nanostructures. At a low temperature of -15 °C, the compound self-assembled into fibrous structures, whereas it formed distinctive flat microbelts at room temperature. When the organogel with nanofibers formed at -15 °C was brought into an ambient condition, chiral twist nanostructures were immediately evolved, which subsequently transferred to a giant microbelt through a hierarchical dendritic twist with the time. Although the compound is achiral, it formed chiral twist with both left- and right-handed twist structures simultaneously. When a trace analogical chiral trigger, L-alanine or D-alanine derivative, was added, a complete homochiral dendritic twist was obtained. Interestingly, a reverse process, i.e. the transformation of the microbelts into twists, could occur upon dilution of the organogel with microbelt structure. During the dilution, both left- and right-handed chiral twists could be formed again. Interestingly, the same branch from the microbelt formed the twist with the same handedness. A combination of the density functional theory (DFT), molecular mechanics (MM), and molecular dynamics (MD) simulations demonstrates that the temperature-induced twisting of the bilayer is responsible for the morphological transformation and evolution of the dendrite twist. This research sheds new light on the hierarchical transformation of the chiral structures from achiral molecules via controlled self-assembly.

Self-instability and bending behaviors of nano plates

This paper aims at investigating the size-dependent self-buckling and bending behaviors of nano plates through incorporating surface elasticity into the elasticity with residual stress fields. In the absence of external loading, positive surface tension induces a compressive residual stress field in the bulk of the nano plate and there may be self-equilibrium states corresponding to the plate self-buckling. The self-instability of nano plates is investigated and the critical self-instability size of simply supported rectangular nano plates is determined. In addition, the residual stress field in the bulk of the nano plate is usually neglected in the existing literatures, where the elastic response of the bulk is often described by the classical Hooke’s law. The present paper considered the effect of the residual stress in the bulk induced by surface tension and adopted the elasticity with residual stress fields to study the bending behaviors of nano plates without buckling. The present results show that the surface effects only modify the coefficients in corresponding equations of the classical Kirchhoff plate theory.

Experimental and theoretical investigations of evaporation of sessile water droplet on hydrophobic surfaces

Experiments of sessile water droplet evaporation on both polydimethylsiloxane (PDMS) and Teflon surfaces were conducted. All experiments begin with constant contact area mode (the initial contact angle is greater than 90°), switch to constant contact angle mode and end with mixed mode. Based on the assumptions of spherical droplet and uniform concentration gradient, theoretical analyses for both constant contact area and constant contact angle modes are made and theoretical solutions are derived accordingly, especially a theoretical solution of contact angle is presented first for CCR stage with any value of the initial contact angle. Moreover, comparisons between the theoretical solutions and experimental data of contact angle in CCR stage demonstrate the validity of the theoretical solution and it would help for a better understanding and application of water droplet on solid surfaces, which is quite often encountered in lab-on-a-chip, polymerase chain reaction (PCR) and other micro-fluidics devices.

Stability and bifurcation behaviour of electrostatic torsional NEMS varactor influenced by dispersion forces

The influences of Casimir and van der Waals forces on the nano-electromechanical systems (NEMS) electrostatic torsional varactor are studied. A one degree of freedom, the torsional angle, is adopted, and the bifurcation behaviour of the NEMS torsional varactor is investigated. There are two bifurcation points, one of which is a Hopf bifurcation point and the other is an unstable saddle point. The phase portraits are also drawn, in which periodic orbits are around the Hopf bifurcation point, but the periodic orbit will break into a homoclinic orbit when meeting the unstable saddle point.

Elastic deformation of soft membrane with finite thickness induced by a sessile liquid droplet

In this paper, the role of vertical component of surface tension of a droplet on the elastic deformation of a finite-thickness flexible membrane was theoretically analyzed using Hankel transformation. The vertical displacement at the surface was derived and can be reduced to Lesters or Rusanovs solutions when the thickness is infinite. Moreover, some simulations of the effect of a liquid droplet on a membrane with a finite thickness were made. The numerical results showed that there exists a saturated membrane thickness of the order of millimeter, when the thickness of a membrane is larger than such a value, the membrane can be regarded as a half-infinite body. Further numerical calculations for soft membrane whose thickness is far below the saturated thickness were made. By comparison between the maximum vertical displacement of an ultrathin soft membrane and a half-infinite body, we found that Lesters or Rusanovs solutions for a half-infinite body cannot correctly describe such cases. In other words, the thickness of a soft membrane has great effect on the surface deformation of the ultrathin membrane induced by a liquid droplet.