Lingyun Wang

Spectroscopic evidence for a high fraction of ferroelectric phase induced in electrospun polyvinylidene fluoride fibers

Effective transformation from paraelectric to a high fraction of ferroelectric phase is crucial to produce piezoelectric materials with a high piezoelectric constant for broad applications. In polyvinylidene fluoride (PVDF) thin films, both mechanical stretching and electric poling processes have been found to be critical in the α → β phase transformation. However, in PVDF fibers fabricated by the electrospinning process, the roles of mechanical stretching and electric poling have not been well explored. Here, the properties of PVDF fibers from electrospinning and forcespinning, a mechanical spinning process without electric poling, have been characterized and analyzed by FTIR and XRD spectroscopic techniques. The results show that pure mechanical stretching in the forcespun fibers can result in a high fraction of the all-trans β-phase, at 95%. Electrospun fibers from the same material system, on the other hand, can also reach a high fraction of β-phase, at approximately 99%. These results preliminarily demonstrate that mechanical stretching is the main reason for β-phase induction in PVDF fibers. Further experiments performed in this work show that higher wt% of PVDF, lower polymer solution supply rate, and more uniformly mixed solvent systems facilitate achieving a higher level of ferroelectric β-phase in electrospun PVDF fibers.

Design and Experiment of a Jetting Dispenser Driven by Piezostack Actuator

To make up for the insufficiency and instability of contact dispensers that are used for fluid dispensing in microelectronic packaging, a noncontact jetting dispenser driven by a piezostack actuator is introduced in this paper. After describing the structural components and operating principle of the dispensing mechanism, a fluid model is presented to discuss the dynamic properties of the fluid and analyze the key parameters of the proposed dispenser. The ANSYS simulation software is used to design the displacement amplifier, which is an important component of the dispenser. The maximum displacement output of 323 μm is obtained by optimizing. Subsequently, the dynamic behavior of the displacement amplifier is measured by an optical displacement sensor. The displacement change with the driving voltage amplitude and frequency is also investigated; the maximum displacement is 320 μm, and the error between the simulation and the measurement result is just 0.75%. In order to verify the practicality of dispenser, experiments are conducted to examine the effects of the driving voltage, backpressure, working temperature, and distance between the nozzle and the collector on the jetting performance and droplet diameter. The dispenser can dispense droplets uniformly and steadily. Its maximum jetting frequency is 65 Hz, and droplets of 1.07-mm diameter are produced by a stainless steel nozzle of 0.25-mm diameter in the experimental study, with the variation of the droplet diameter being within ±2%.

Electrospinning-induced preferred dipole orientation in PVDF fibers

Polyvinylidene fluoride (PVDF) can be made electroactive by properly mechanical stretching and electric poling treatments of its film, which may be easily realized by single-step electrospinning. This technique is acknowledged as an effective approach to induce rich ferroelectric β-phase in electrospun PVDF fibers; however, the investigation of dipole arrangement during the electrospinning process is still lacking. Here, the piezoelectricity of β-PVDF fibers by electrospinning and forcespinning, a mechanical spinning process without static electric field bias, has been demonstrated. Results show that the electrospun fibers can generate piezoelectric voltage after deformation, while the forcespun fibers nearly show no piezoelectricity for the same condition, revealing that electric field during the electrospinning process can perform in situ poling effect and therefore induces preferred dipole orientation in electrospun PVDF fibers. Further experiments performed in this work show that piezoelectricity of the electrospun fibers increases with increasing fraction of β-phase and/or the applied electric field strength of electrospinning, which provides good guideline for preparing high-performance piezoelectric fibers.

Simulation and experiment study on adhesive ejection behavior in jetting dispenser

In this paper, fundamental equations for jetting dispenser are presented to express the influence of adhesive pressure, nozzle diameter, and needle movement law, and the equations are then verified by flow behavior simulation. Subsequently, a novel jetting dispenser system is built to finish experiments about the mentioned influence parameters, and simulation results verify the regularities from experiment are correct. Flow velocity in the central nozzle will be faster and radius of the droplet will be bigger if the pressure in the chamber became higher, flow velocity in the central nozzle will be constant and the radius of the droplet will be bigger if the dead time became longer, and bigger nozzle diameter can lead to faster flow velocity in the central nozzle and bigger droplet. Besides, these mentioned works prove that our designed jetting dispenser is practical and useful for adhesive jetting.

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.

Development of a Microforce Sensor and Its Array Platform for Robotic Cell Microinjection Force Measurement

Robot-assisted cell microinjection, which is precise and can enable a high throughput, is attracting interest from researchers. Conventional probe-type cell microforce sensors have some real-time injection force measurement limitations, which prevent their integration in a cell microinjection robot. In this paper, a novel supported-beam based cell micro-force sensor with a piezoelectric polyvinylidine fluoride film used as the sensing element is described, which was designed to solve the real-time force-sensing problem during a robotic microinjection manipulation, and theoretical mechanical and electrical models of the sensor function are derived. Furthermore, an array based cell-holding device with a trapezoidal microstructure is micro-fabricated, which serves to improve the force sensing speed and cell manipulation rates. Tests confirmed that the sensor showed good repeatability and a linearity of 1.82%. Finally, robot-assisted zebrafish embryo microinjection experiments were conducted. These results demonstrated the effectiveness of the sensor working with the robotic cell manipulation system. Moreover, the sensing structure, theoretical model, and fabrication method established in this study are not scale dependent. Smaller cells, e.g., mouse oocytes, could also be manipulated with this approach.

Predicting Polymorphism of Electrospun Polyvinylidene Fluoride Membranes by Their Morphologies

Although electrospinning of polyvinylidene fluoride (PVDF) has been studied for more than 10 years, the crystalline phase differentiation of the electrospun mats is still normally through the combination of different characterization techniques, and the relationship between polymorphism and morphology of the fibers in electrospun PVDF membranes has never been reported. Here, we show their close relationships by conducting room-temperature electrospinning experiments on various polymer/solvent systems. The electrospun membranes full of bead-free fibers have a very high fraction of β-phase, F(β), over 90%, and high orientation, whereas the membranes comprising beads and/or a large number of beaded fibers most often result in a low fraction of β-phase (F(β) normally below 50%) and low orientation. On the other hand, electrospun membranes consisting of both bead-free fibers and a very limited number of beaded fibers showed a medium high fraction of β-phase, F(β) more than 70% but less than 90%. These findings suggest the feasibility of intuitively predicting the crystalline phase of electrospun PVDF membranes directly by their morphologies, which is obviously simple, inexpensive and convenient for future investigations.

A novel sacrificial-layer process based on anodic bonding and its application in an accelerometer

It is found in our experiments that the depletion layer of anodic bonding is etched faster than the bulk glass (Pyrex 7740) in hydrofluoric acid (HF). Based on this interesting phenomenon, a novel process of a sacrificial layer is proposed in this paper. In order to deeply understand and investigate the rules concerning the influence of bonding parameters on this effect, firstly the width of the depletion layer under different bonding voltages and temperatures and the selection ratio of etching are revealed. To validate the feasibility of the method, a micro-machined accelerometer is designed and fabricated. The test results of resonant frequency and sensitivity of the fabricated accelerometer are 3254.5 Hz and 829.85–844.93 mV/g, respectively. This was further evidence that the depletion layer could be used as a sacrificial layer and the removable structure could be successfully released by fast etching this layer. The important feature of this method is that only one mask is needed in the whole process and therefore it could greatly simplify the fabrication process of the device.

Direct-Write micro/nano-structure for flexible electronic manufacturing

This article focuses on optimizing the electrospinning parameters, and developing a new method of Direct-Write (DW) micro/nano-structure based on Near-Field Electrospinning (NFES) for flexible electronic manufacturing. NFES is a new way to realize controllable electrospinning and precision-positioning of nanofiber, by which nano-structure with diameter from 50 nm to 500 nm can be fabricated orderly and accurately. A tungsten electrode with tip diameter of 25 mum is used to DW nano-structure, with the minimum bias voltage 600 V, minimum electrode to collector distance 500 mum. A micro-structure DW system is designed, by which micro-structure with diameter of several micrometers can be drawn. In this work, a needle tube of 232 mum inside diameter is used as spinneret, electrode to collector distance is various from 2 mm to 10 mm, and the collector moving speed ranges from 0.07 m/s to 7 m/s. The DW process and character of micro-structure such as line width, smoothness and thickness can be controlled by optimizing the electrospinning parameters. The DW micro/nano-structure is continuous and smooth, which can be drawn on expected site and in expected direction with accurate dimension. The new method based on NEFS with the advantage of narrower line width and smoother structure than traditional flexible electronic manufacturing technologies, which is more suitable for the development of flexible electronic manufacturing.

Electrospun Nanofibers Bundles

Aligned nanofibers, filament bundle composed of large number of nanofibers have potential applications such as biomaterials, composite materials etc. A series of electrospinning experiment has been conducted to investigate the electrospinning process, through which, some parameters such as polymer solution concentration, bias voltage, distance between spinneret and collector, solution flow rate etc have been setup to do the experiment of nanofibers bundles construction. This work firstly reports electrospun nanofibers bundles through non-uniform electrical field, and nanofibers distributed with different density on electrodes from that between them. Thinner nanofibers bundle with a few numbers of nanofiber is collected for 3 seconds; therefore its also possible that the addressable single nanofiber could be collected bridging the two electrodes