Nanofluidic Diode for Simple Fluids without Moving Parts

The fabrication of small scale, fixed structure fluidic diodes for simple fluids is quite challenging and has not yet been achieved. Here, we fabricate a moving part-free nanofluidic diode for simple fluids using heterogeneous nanochannels, half of which is hydrophilic and the other half is hydrophobic. It accepts water flows in the forward (from hydrophilic to hydrophobic) direction, while the flows in the backward direction are blocked for pressure drop range $0<\mathrm{\Delta }P<0.63\mathrm{MPa}$. The diode is ensured by a potential energy barrier at the channel entrance on the hydrophobic side due to the molecular interactions between the water and channel surface. As the upstream pressure becomes higher than 0.63 MPa, the fluidic diode turns to be a rectifier, which allows flows in both the forward and backward directions but with different flow rates. At sufficiently high driving pressures, the fluidic system fails in flow rectification, analogous to the breakdown of electronic diodes. The three different flow modes (diode, rectifier, and breakdown) of the fluidic chip and the underlying rectification mechanisms are confirmed by molecular dynamics simulations.

Figure 1

Schematic of the nanofluidic system.

Figure 2

Fabrication process for the nanofluidic chip (dimensions are not in scale). (a) Growth of 5-nm-thick ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer on ${\mathrm{SiO}}_2$-coated silicon wafer through photolithography and wet etching. The ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer was subsequently modified by low surface energy material (red). (b) Deposition of silicon sacrificial layer. (c) Fabrication of another ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer serving as the top wall of the hydrophobic part. (d) Wrapping with ${\mathrm{SiO}}_2$ layer through plasma-enhanced chemical vapor deposition. (e) Reactive ion etching of microchannels and reservoirs (not shown). (f) Anodic bonding of glass cover and removal of sacrificial layer.

Figure 3

Nanochannel characterization. Top panel: SEM image showing the height (97.8 nm) and width ($10\mu \mathrm{m}$) of a nanochannel. Bottom panel: microscopic image of a fluorescent flow through the fluidic chip.

Figure 4

Experimental setup of the fluidic system. DI water is driven by a syringe pump with a desired flow rate. Arrows indicate the directions of forward flows. Pressures of the reservoirs $P_1-P_4$ are measures by pressure transducers.

Figure 5

Total volumetric flow rate and flux through the fluidic chip as a function of $\mathrm{\Delta }P$. The solid line is the prediction of the Navier-Stokes equations. Error bars denote the standard deviation of three experiments. Diodicity in the inset represents the pressure drop ratio between the forward and backward directions.

Figure 6

A snapshot of the molecular dynamics simulation system. The left and right halves of the channel are hydrophilic and hydrophobic.

Figure 7

Potential energy distribution in an $x\text{−}y$ plane near the bottom surface ($y=0$) of the nanochannel in MD simulations. The hydrophilic and hydrophobic entrances are at $x=90$ and 168 Å, respectively. The inset shows the potential energies along $y=2$ and 4 Å.

Figure 8

Volumetric flux obtained in the MD simulations. Diodicity in the inset denotes the flux ratio.