Micro Mechanical Mechanism Of Water Transport And Ions Sieving Through Nanochannels

Water transport and ions sieving through nanochannels are widely adopted in water desalination,biomimetic technology and nanodevice fabrication.Studies on the micro mechanism of water transport and ions sieving through nanochannels play an important role in the subjects of physics,chemistry,mechanics,life science and material science.In this dissertation,three core mechanical problems were chosen and investigated systematically:the boundary slip,the contact angle hysteresis and the size effect.A theoretical model extended from the molecular kinetic theory(MKT)was proposed to describe the boundary slip on textured surfaces.The concept of the equivalent depth of potential well was adopted to characterize the solid-liquid interactions on the textured surfaces.The slip behaviors on both chemically and topographically textured surfaces were investigated using molecular dynamics(MD)simulations.The nonlinear increase of slip velocity under high shear stress can be predicted using this extended slip model,which also provides a microscopic interpretation about the influence of the surface corrugation on the interfacial flow.The unidirectional droplet transport sandwiched between non-parallel smooth and saw-tooth plates based on contact angle hysteresis analysis was investigated.The repeated opening and closing of the plates result in sequential stretching and squeezing of the droplet,which provides the driving force.The asymmetrical saw tooth obstructs the backward motion of the droplet,and gives rise to a net forward displacement of the droplet in every opening and closing cycle.By virtue of the ratchet-like effect,the saw-tooth plates can not only hinder the backward droplet motion on hydrophobic plates,but also greatly increase the transport efficiency.It is verified that contact line pining is not necessary for unidirectional droplet transport.The droplet transport can be achieved through the accumulation of a net displacement towards the model apex in every circle.A nano screw pump(NSP)was designed with a helical nanowire embedded inside a nanochannel,which was used for driving unidirectional water flow.By investigating the water transport mechanism through nano screw pumps with different configuration parameters,three transport modes were observed:cluster-by-cluster,pseudo-continuous,and linear-continuous.The influence of the nanowires' rotating frequency,surface wettability and screw's diameter was investigated.Results showed that the water flux rate increases as the decreasing wettability of helical nanowires.The weak hydrogen bonding due to space confinement and the hydrophobic blade in smaller screw pumps aggravates the nonlinear water transport.Water desalination using NSP was studied.With narrow screw pitch,sodium chloride(NaCl)can be effectively filtered from water based on the size effect.The mechanisms of water transport through NSPs can be explained by the synergistic effect of confinement resistance and pitch-dependent water transport modes.The water permeation can be improved by increasing the NSP diameter and decreasing the screw pitch number.The salt rejection capacity and superior water permeation has proved the NSP an excellent nanodevice for not only water desalination,but also more general liquid separation based on space confinement.