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In the narrow orifice of a cone-shaped channel, blockage can occur for granular matter. However, water molecules can enter into and even permeate through carbon nanotubes of diameters down to 0.8 nm at ultrafast rates. Here we demonstrate by molecular dynamics simulations that clogging can also emerge unexpectedly in the water flowing through a nanoscale valve-less pump. The designed pump features two truncated carbon nanocones, with the narrowest region having a diameter of 1.2 nm (larger than that of (6, 6) carbon nanotube), linked to a fluid cavity volume, and is powered by the vibration of a graphene sheet. In the low frequency range, water molecules can be driven through the nanocones effectively by the vibration of the graphene sheet. The maximum flux reaches 83
$ {{\mathrm{n}}{\mathrm{s}}}^{-1} $ , which is approximately 20 times the measured value of$ (3.9\pm 0.6){{\mathrm{n}}{\mathrm{s}}}^{-1} $ for aquaporin-1. However, at higher frequencies, water molecules suffer blockage at the narrow exits. Much unlike granular matter, high-frequency vibration cannot restore water flow. The key to this phenomenon is that in the narrow exits of two nanocones acting as diffuser/nozzle, the number density of water molecules rapidly increases with frequency increasing, the tight hydrogen-bonding network is formed, and the mean lifetime of hydrogen bonds increases dramatically under high-frequency vibrations. High frequency fluctuations in the middle chamber make H-bond network between water molecules in the narrow exits more stable. The probability density distribution of water exhibits a non-equilibrium transition from a disordered state to ordered state. This work reveals a new mechanism of water flowing/blocking in a nanoscale valve-less pump based on two asymmetrical nanocones, offers valuable insights into understanding nonequilibrium jamming transition in nanoscale fluid.-
Keywords:
- valveless nanopump /
- clogging /
- vibration /
- hydrogen bond network
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图 1 模拟系统图, 由左右两个锥形碳纳米管及中间泵体组成的无阀纳米泵, 通过两个碳平面(灰色小球)连接左右水浴, 红色小球和白色小球分别表示水分子中的氧原子和氢原子; 橘色小球代表振动原子; 左右锥形碳纳米管的窄口端用黄色标记, 分别记为LP (left pore)和RP (right pore)
Fig. 1. Simulation system. A valve-less nanopump consists of carbon nanocones on both sides, a fluid cavity in the center, connected by carbon planes (gray spheres) to two reservoirs. Red and white spheres represent oxygen and hydrogen atoms in water molecules, respectively. The orange sphere represents the vibrating atom. The narrow ends of the carbon nanocones are marked in yellow and labeled as LP (left pore) and RP (right pore), respectively.
图 2 水流量随振动频率的变化关系
Fig. 2. Average water flux as a function of the vibration frequency.
图 3 在锥形纳米管窄口端0.4 nm厚的区域内水分子的概率密度分布图 (a) f = 384 GHz; (b) f = 625 GHz; (c) f = 833 GHz; (d) f = 1250 GHz
Fig. 3. Probability density distributions of water molecules in LP for different vibrational frequency: (a) f = 384 GHz; (b) f = 625 GHz; (c) f = 833 GHz; (d) f = 1250 GHz.
图 4 LP和RP区域的平均水分子数, 两个区域厚度为0.4 nm, 分别包含2层水分子, L1和R1分别表示两个区域中右侧的(管径最小)水分子层; L2和R2则表示左侧的水分子层
Fig. 4. Average number of water molecules in each water layer in LP and RP. L1 and R1 represent the right-side (minimum diameter) water molecule layers in the two regions, while L2 and R2 represent the left-side water molecule layers.
图 5 (a)氢键的平均寿命随频率的变化, 插图分别为f = 250 GHz和f = 1250 GHz对应的 LP里第1层水的模拟快照(青色小球表示碳原子, 红色和白色小球分别表示水分子中的氧原子和氢原子); (b)LP区域第1层水分子在横截面上的方位角概率分布图
Fig. 5. (a) Lifetime of hydrogen bonds as a function of the vibration frequency. Two typical snapshots are shown in insets. The cyan spheres represent carbon atoms, with red and white spheres representing oxygen and hydrogen atoms in water molecules. One is for the low frequency (f = 250 GHz) and the other is for the high frequency (f = 1250 GHz). (b) Probability distribution of the azimuthal angle of water molecules in the first layer in the LP region on the cross-section for f = 250 GHz and f = 1250 GHz.
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