Guohui Hu

Three dimensional flow structures in a moving droplet on substrate: A dissipative particle dynamics study

It is of both fundamental and practical interest to study the flow physics in the manipulation of droplets. In this paper, we investigate complex flow in liquid droplets actuated by a linear gradient of wettability using dissipative particle dynamics simulation. The wetting property of the substrate ranging from hydrophilic to hydrophobic is achieved by adjusting the conservative solid-liquid interactions which results in a variation of solid-liquid surface tension. The internal three-dimensional velocity field with transverse flow in droplet is revealed and analyzed in detail. When the substrate is hydrophobic, it is found that there is slight deformation but strong flow circulation inside the droplet, and the droplet rolling is the dominant mechanism for the movement. However, large deformation of the droplet is generated after the droplet reaches the hydrophilic surface, and a mechanism combining rolling and sliding dominates the transportation of the droplet. Another interesting finding is that the th...

Water structures inside and outside single-walled carbon nanotubes under perpendicular electric field

The structures of water inside and outside (6,6), (8,8), and (10,10) singlewalled carbon nanotubes (SWCNTs) under an electric field perpendicular to the tube axis are investigated by molecular dynamics simulations. The results show that dipole reorientation induced by electric field plays a significant role on the structures of confined water inside and outside SWCNTs. Inside SWCNTs, the average water occupancy and the average number of hydrogen bonds (H-bonds) per water molecule decrease as the electric intensity increases. Because the field intensity is sufficiently strong, the initial water structures inside the SWCNTs are destroyed, and the isolated water clusters are found. Outside SWCNTs, the azimuthal distributions of the density and the average number of H-bonds per water molecule around the solid walls become more and more asymmetric as the electric intensity increases. The percentages of water molecules involved in 0-5 H-bonds for all the three types of SWCNTs under different field intensities are displayed. The results show that those water molecules involved with most H-bonds are the most important to hold the original structures. When the electric field direction is parallel with the original preferred orientation, the density and the H-bond connections in water will be increased; when the electric field direction is perpendicular to the original preferred orientation, the density and the H-bond connections in water will be decreased.

Manipulation of a neutral and nonpolar nanoparticle in water using a nonuniform electric field

The manipulation of nanoparticles in water is of essential importance in chemical physics, nanotechnology, medical technology, and biotechnology applications. Generally, a particle with net charges or charge polarity can be driven by an electric field. However, many practical particles only have weak and even negligible charge and polarity, which hinders the electric field to exert a force large enough to drive these nanoparticles directly. Here, we use molecular dynamics simulations to show that a neutral and nonpolar nanoparticle in liquid water can be driven directionally by an external electric field. The directed motion benefits from a nonuniform water environment produced by a nonuniform external electric field, since lower water energies exist under a higher intensity electric field. The nanoparticle spontaneously moves toward locations with a weaker electric field intensity to minimize the energy of the whole system. Considering that the distance between adjacent regions of nonuniform field intensity can reach the micrometer scale, this finding provides a new mechanism of manipulating nanoparticles from the nanoscale to the microscale.

Dissipative Particle Dynamics Simulation of Droplet Oscillations in AC Electrowetting

Abstract A mesoscopic model is developed to investigate the oscillations of a sub-micrometer droplet in AC electrowetting based on dissipative particle dynamics. To simulate the effects of the applied AC voltage, we vary the interaction between the solid and liquid particles aiming to recover the contact angles obeying the Lippmann–Young equation. The low frequency flow obtained in the present study is consistent qualitatively with previous experimental measurement. For the intermediate frequency voltage, generally no significant movement is found inside the droplet except fluid oscillatory motion near the contact line. The contact line dynamics is investigated as well, in which the results show the hysteresis phenomenon of contact line movement, and the phase difference between the applied voltage variation and the contact line oscillation generally increases with the AC frequency except for some resonance frequencies. Furthermore, the amplitude of the contact line is found to decrease linearly on a logarithmic scale with the applied frequency.

Guided motion of short carbon nanotube driven by non-uniform electric field

The molecular dynamics simulations are performed to show that in aqueous environments, a short single-walled carbon nanotube (SWCNT) guided by a long SWCNT, either inside or outside the longer tube, is capable of moving along the nanotube axis unidirectionally in an electric field perpendicular to the carbon nanotube (CNT) axis with the linear gradient. The design suggests a new way of molecule transportation or mass delivery. To reveal the mechanism behind this phenomenon, the free energy profiles of the system are calculated by the method of the potential of mean force (PMF).

Nanoscale cavitation in perforation of cellular membrane by shock-wave induced nanobubble collapse

The collapse of the bubble induced by the shock wave leads to nano-jet, which is able to perforate cellular membranes. This phenomenon is investigated by Martini coarse-grained molecular dynamic (CG-MD) simulations in the present study. It is found that the occurrence of cavitation nucleation at the nanoscale can be observed during the perforation process. The cavitation locates near the puncture of the cell membrane and its ultimate evolutionary form presents a ring-like structure. The volume of the cavitation is calculated for different initial bubble sizes, and it is found that the maximum volume of the cavitation area has a correlation with the initial bubble size. To understand the underlying physics of the cavitation phenomenon, the classical nucleation theory based on the Rayleigh-Plesset equation is applied to the non-equilibrium nanoscale system after the pressure field is obtained by using the Irving-Kirkwood-Noll procedure. The consistence between the results of CG-MD and the theory reveals that the average pressure of the local environment plays a crucial role in cavitation occurrence on the non-equilibrium system subjected to strong inertia, e.g., shock wave and nano-jet.