Electrohydrodynamic Direct-Writing Micropatterns with Assisted Airflow

Recent advancements in physical and chemical MEMS sensors

Microelectromechanical systems (MEMSs) are microdevices fabricated using semiconductor-fabrication technology, especially those with moving components. This technology has become more widely used in daily life, e.g., in mobile phones, printers, and cars. In this review, MEMS sensors are largely classified as physical or chemical ones. Physical sensors include pressure, inertial force, acoustic, flow, temperature, optical, and magnetic ones. Chemical sensors include gas, odorant, ion, and biological ones. The fundamental principle of sensing is reading out either the movement or electrical-property change of microstructures caused by external stimuli. Here, sensing mechanisms of the sensors are explained using diagrams with equivalent circuits to show the similarity. Examples of multiple parameter measurement with single sensors (e.g. quantum sensors or resonant pressure and temperature sensors) and parallel sensor integration are also introduced.

Flexible Patterned Electrohydrodynamic Jet Printing Using Orthogonal Deflection Electrodes

Electrohydrodynamic jet (E-Jet) printing technology provides unmatched advantages in the fabrication of patterned micro/nanostructures. However, the rapid jets generated during printing can lead to localized droplet accumulation on complex structures due to the relatively slow motion control achieved with motorized translation stages, resulting in distorted patterns. To address this challenge, we introduce two jet-deflecting electrodes orthogonally placed on each other, which can rapidly change the electric field in the vicinity of the jet and thus flexibly adjust the flight trajectory of the fast jet to avoid the region where droplets have been deposited. In this way, the jet droplets are precisely controlled to generate high-fidelity microstructures with arbitrary predefined patterns on the stationary substrate. The maximum deflection distance of the jet droplets reaches several hundred microns. Furthermore, the positioning error of the printed structure is less than 3%. Moreover, we successfully obtained a diverse range of complex patterns by combining this technique with stage motion. This innovative printing technology not only enables the fabrication of complex patterned structures with high fidelity but also opens up exciting possibilities for new applications that require complete control of fast droplet positioning.

Beyond the Single-Nozzle: Coaxial Electrospinning Enables Innovative Nanofiber Chemistries, Geometries, and Applications

With an ever increasing scientific, technological, and industrial interest in high surface area, porous nanofiber mats, electrospinning has emerged as a popular method to produce fibrous assemblies for use across biomedical, energy, and environmental applications. However, not all precursor solutions nor complex geometries can be easily fabricated using the traditional single-nozzle apparatus. Therefore, coaxial electrospinning, a modified version of electrospinning that features a concentrically aligned dual nozzle, has been developed. This review will first describe the mechanism of electrospinning two precursor solutions simultaneously and the operational parameters that need to be optimized to fabricate continuous fibers. Modifications that can be made to the coaxial electrospinning process, which enable the fabrication of uniform fibers with improved properties, as well as the fabrication of fibers that are hollow, functionalized, and from "nonspinnable precursors" will be discussed as a means of promoting the advantages of using a coaxial setup. Examples of how coaxially electrospun nanofibers are employed in diverse applications will be provided throughout this review. We conclude with a timely discussion about the current limitations and challenges of coaxial electrospinning.

Dripping, jetting and tip streaming

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.

Microlens Fabrication by Replica Molding of Electro-Hydrodynamic Printing Liquid Mold

In this paper, we synergistically combine electrohydrodynamic (EHD) printing and replica molding for the fabrication of microlenses. Glycerol solution microdroplets was sprayed onto the ITO glass to form liquid mold by an EHD printing process. The liquid mold is used as a master to fabricate a polydimethylsiloxane (PDMS) mold. Finally, the desired micro-optical device can be fabricated on any substrate using a PDMS soft lithography mold. We demonstrate our strategy by generating microlenses of photocurable polymers and by characterizing their optical properties. It is a new method to rapidly and cost-effectively fabricate molds with small diameters by exploiting the advantages of EHD printing, while maintaining the parallel nature of soft-lithography.

Jet Mode Recognition of Electrohydrodynamic Direct-Writing Based on Micro/Nano Current

The online recognition of jet mode is important for the accurate control and further application of electrohydrodynamic direct-writing (EDW) technology. An EDW system with a current detection module is built for jet mode recognition. The current of the EDW jet is measured to recognize the jet mode when printing patterned structures. Then, a data processing program with a digital Kaiser low-pass filter is developed in MATLAB, via which the noise of the current signal is reduced. The features of EDW current, including the current fluctuation and the peak current intervals, are studied to recognize different jet modes. The current characteristics of three jet modes are investigated: droplet ejection mode, Taylor cone ejection mode, and retractive ejection mode. The Taylor cone ejection mode has the smallest coefficient of variation of peak current. This work provides a good way of designing the optimized control algorithm and of realizing the closed-loop control system, which contributes to enhancing the jet stability and accelerating the application of EDW technology.

Measurement and Time Response of Electrohydrodynamic Direct-Writing Current

The micro/nano current is an important characteristic to reflect the electrohydrodynamic direct-writing (EDW) process. In this paper, a direct-written current measurement system with a high signal to noise ratio was proposed to monitor the charged jets, providing the data basis for the promotion of stability and precision of the EDW jet. The electrical characteristics of the printing process were studied, the electrohydrodynamic direct-written current was associated with the stability of charged jet and the accuracy of direct-written patterns. There was an impulse current when the front end of the jet reached the collector and then a stable jet could be gained. With the increase of applied voltage, the severe fluctuation of measured current increased, the charged jet became more unstable and the accuracy of direct-written parallel lines was lower. The effects of processing parameters on direct-written current were also investigated. The average direct-written current at the stable stage increased as the applied voltage and polymer concentration increased, and it decreased as the distance from the nozzle to the collector increased. This research will promote the development and applications of EDW technology in the fields of micro/nano manufacturing.