Wenwang Li

Precision deposition of a nanofibre by near-field electrospinning

The deposition behaviour of an individual nanofibre on planar and patterned silicon substrates is studied using near-field electrospinning (NFES). A high-speed camera was utilized to investigate the formation and motion process of a liquid jet. Thanks to the shorter distance from the spinneret to the collector, bending instability and splitting of the charged jet in electrospinning were overcome. In NFES, a straight-line jet between the spinneret and the collector can be utilized to direct-write an orderly nanofibre. Perturbation stemming from residual charges on the collector caused the oscillation of the charged jet, and the deposition of the non-woven nanofibre on the planar substrate. With increasing collector speed, the impact of residual charges was weakened by the strong drag force from the collector and a straight-line nanofibre could be obtained. In addition, the nanofibre can be direct-written in a special pattern by controlling the motion track of the collector. Therefore, it can be concluded that a micro-strip pattern was a good guidance for nanofibre deposition, and the nanofibre deposition track followed well along the top surface of the micro-strip pattern. The position-controlled deposition of a single nanofibre provides a new aspect for applications of electrospinning.

Experiment and simulation of coiled nanofiber deposition behavior from near-field electrospinning

Both experiment and simulation works were utilized to research the deposition behavior of single coiled nanofiber on silicon substrate from Near-Field Electrospinning (NFES). In the experiment process, when the collector moving speed (CMS) is compatible with electrospinning speed, straight line nanofiber can be collected. When CMS decreases, jet would step into whipping motion due to the imbalance charge repulsive force from landed nanofiber. As decreasing CMS, nanofiber in waved shape, single circle coil and multi-circle coil can be fabricated in turn. In order to improve the controlling technology, a computational model based on Maxwell viscoelastic theory was build up to analyzed the deposition behavior of single nanofiber. Simulation results show that CMS is the main controlling parameter influence the nanofiber deposition morphology: beads deposited in straight line, when CMS higher than 0.35m/s; beads deposited in waved shape, when CMS lies between 0.15m/s and 0.35m/s; beads in single-circle coiled can be gained, when CMS ranges from 0.08m/s to 0.15m/s; beads would deposit in multi-circle coil, when CMS lower than 0.08m/s. The effect of collector conductivity was also investigated by the computational modeling: the diameter of multi-circle nanofiber zone and distance between adjacent nanofiber coil increases with collector conductivity decrease. The calculated behaviors of nanofiber deposition are in good agreement with the experimental results, which is a good way to represent the motion behavior of charged jet and nanofiber.

Directly electrospun ultrafine nanofibres with Cu grid spinneret

A hydrophobic Cu grid was used as an electrospinning spinneret to fabricate ultrafine organic nanofibres. The Cu grid used in this study was that which holds samples in TEM. Due to the hydrophobic surface and larger contact angle of the electrospinning solution on the Cu grid surface, the solution flow was divided into several finer ones by the holes in the Cu grid instead of accumulating. Each finer flow was stretched into individual jets and established a multi-jet mode by the electrical field force. The finer jets played an important role in decreasing the diameter of the nanofibre. The charge repulsion force among charged jets enhanced the whipping instability motion of the liquid jets, which improved the uniformity of the nanofibre and decreased the diameter of the nanofibre. An ultrafine uniform nanofibre of diameter less than 80 nm could be fabricated directly with the novel Cu grid spinneret without any additive. This study provided a unique way to promote the application of one-dimensional organic nanostructures in micro/nanosystems.

A novel bulk micromachined tunneling gyroscope

A bulk micromachined vibratory tunneling gyroscope, which employs the high displacement sensitivity of quantum tunneling to obtain the desired resolution and is fabricated with silicon-glass wafer bonding and DRIE (Deep Reactive Ion Etching), has been developed. The device structure consists of a proof mass which can oscillate due to electrostatic comb driving and an out-of-plane silicon frame linked up to substrate by suspended springs. Because of adopting the silicon frame structure to get larger proof mass and putting tunneling tip at the end of silicon frame to obtain remarkable deformation induced by Coriolis force, the new ultracompact devices can provide extremely high sensitivity and wide dynamic range. Based on the modal analysis of gyroscope by FEM method, the structure dimensions are optimized according to resonant frequency matching of driving mode and detection mode. Simulation results demonstrate that the gyroscope owns the sensitivity of 0.7 Å(°/sec) at atmospheric pressure. The deep dry silicon on glass (DDSOG) process has been successfully used to fabricate this bulk tunneling gyroscope. The tunneling current is observed at deflection voltage 45.7V. The exponential relationship between tunneling current and square deflection voltage verifies the tunneling effect mechanism between the tip and the detected electrode.

Single-step fabrication of organic nanofibrous membrane for piezoelectric vibration sensor

A modified electrospinning setup with parallel electrode and heater plate was presented to fabricate piezoelectric nanofibrous membrane directly. Thanks to the strong electric field strength, high stretch ratio and high environmental temperature during the electrospinning process, α-phase poly (vinylidene fluoride) (PVDF) would be transformed to β-phase well and have excellent piezoelectric performance. The maximal piezoelectric output voltage increased from 100 mV to 300 mV with increasing excitation frequency from 5 Hz to 1000 Hz, and the noise in the piezoelectric response decreased with increasing excitation frequency. This work would promote the application of polymeric sensor in the NEMS/MEMS system integration.

Buckling nanofiber on patterned substrate from near-field electrospinning

Near-Field Electrospinning (NFES) was utilized to research the buckling behaviors of electrospinning nanofiber on the silicon patterned substrate. Nanofiber would be deposited in spiral shape due to the bulking behaviors of jet. Buckling of jet is more obvious under the stronger electrical field and lower collector motion speed (CMS), which leads nanofiber to be deposited in spiral shape. The electrical field strength above the micro-pattern is stronger than other zone, which would enhance the buckling behaviors of charged jet and accelerate the motion speed of nanofiber. Higher CMS is needed to direct-write straight line nanofiber on the top surface of micro-pattern than that on the other zone: nanofiber would be deposited in straight line on all over the surface of the patterned substrate, when CMS is higher than 0.3m/s; nanofiber deposited in spiral shape on the top surface of micro-pattern but in straight line on the surface of other zone, when CMS ranging from 0.1m/s to 0.3m/s; spiral nanofiber deposited all over the surface of patterned substrate, when CMS is lower than 0.1m/s. Furthermore, nanofiber deposited on the top surface of micro-pattern is stretched more and have smaller diameter. Due to the edge effect of electrical field and elastic force of nanofibers, more nanofibers would be collected on the edge or corner of micro-pattern on the substrate. There is an interesting phenomena, multi-layer coiled nanofiber would be collected nearby the edge of micro-pattern, when CMS is lower than 0.08m/s. The work would improve the integration of NFES with other micro/nano-fabrication technology.