Electrohydrodynamic Patterning of Polyethersulfone Membranes

Rapid In-Plane Pattern Growth for Large-Area Inverse Replication Through Electrohydrodynamic Instability of Polymer Films

Nanopatterning driven by electrohydrodynamic (EHD) instability can aid in the resolution of the drawbacks inherent in conventional imprinting or other molding methods. This is because EHD force negates the requirement of physical contact and is easily tuned. However, its potential has not examined owing to the limited size of the pattern replica (several to tens of micrometers). Thus, this study proposes a new route for large-area patterning through high-speed evolution of EHD-driven pattern growth along the in-plane axis. Through the acceleration of the in-plane growth, while selectively controlling a specific edge growth, the pattern replica area can be extended from the micro- to centimeter scale with high fidelity. Moreover, even in the case of nonuniform contact mode, the proposed rapid in-plane growth mode facilitates uniform large-scale replication, which is not possible in conventional imprinting or other molding methods.

Polarization Effect and Improvement of the Electroinduced Formation Efficiency of a Highly Viscous Liquid Bridge

In the electroinduced formation of a highly viscous liquid bridge, improving the efficiency of formation is important for industrial applications. This paper presents the preregulation method of the polarization status to shorten the formation time of a liquid bridge. The hindering effect of high viscosity on the polarization of liquid suspensions was investigated. The formation time of the liquid bridge is shortened, and stability is improved by prepolarizing the initial liquid film, with a maximum reduction in the average and standard deviation of times by 12.65 and 2.52 s, respectively. These effects are confirmed at different viscosities and voltages. In addition, this method has no obvious influence on the shape of the liquid bridge. This study provides an approach to improve the electroinduced formation.

Designing sustainable membrane-based water treatment via fouling control through membrane interface engineering and process developments

Membrane-based water treatment processes have been established as a powerful approach for clean water production. However, despite the significant advances made in terms of rejection and flux, provision of sustainable and energy-efficient water production is restricted by the inevitable issue of membrane fouling, known to be the major contributor to the elevated operating costs due to frequent chemical cleaning, increased transmembrane resistance, and deterioration of permeate flux. This review provides an overview of fouling control strategies in different membrane processes, such as microfiltration, ultrafiltration, membrane bioreactors, and desalination via reverse osmosis and forward osmosis. Insights into the recent advancements are discussed and efforts made in terms of membrane development, modules arrangement, process optimization, feed pretreatment, and fouling monitoring are highlighted to evaluate their overall impact in energy- and cost-effective water treatment. Major findings in four key aspects are presented, including membrane surface modification, modules design, process integration, and fouling monitoring. Among the above mentioned anti-fouling strategies, a large part of research has been focused on membrane surface modifications using a number of anti-fouling materials whereas much less research has been devoted to membrane module advancements and in-situ fouling monitoring and control. At the end, a critical analysis is provided for each anti-fouling strategy and a rationale framework is provided for design of efficient membranes and process for water treatment.

Pattern formation in thin polymeric films via electrohydrodynamic patterning

The free surface of a thin polymeric film is often unstable and deforms into various micro-/nano-patterns under an externally applied electric field. This paper reviews a recent patterning technique, electrohydrodynamic patterning (EHDP), a straightforward, cost-effective and contactless bottom-up method. The theoretical and numerical studies of EHDP are shown. How the characteristic wavelength and the characteristic time depend on both the external conditions (such as voltage, film thickness, template-substrate spacing) and the initial polymer properties (such as rheological property, electrical property and surface tension) is theoretically and experimentally discussed. Various possible strategies for fabricating high-aspect-ratio or hierarchical patterns are theoretically and experimentally reviewed. Aligning and ordering of the anisotropic polymers by EHDP is emphasized. A perspective, including novelty and limitations of the methods, particularly in comparison to some conventional patterning techniques, and a possible future direction of research, is presented.

Loading a High-Viscous Droplet via the Cone-Shaped Liquid Bridge Induced by an Electrostatic Force

In transfer printing, the loaded droplet on the probe has a significant influence on the dispensing resolution. A suitable loading approach for a high-viscous liquid is highly required. Herein, a novel electrostatic loading method is presented, in which the main aim is to control precisely the formation and breaking of a cone-shaped liquid bridge. An experimental device is developed. The influence of electrical and geometric parameters on the feature size of the liquid bridge is investigated in detail. In the formation of the liquid bridge, the increase of voltage or the decrease of the air gap can enhance the electric field intensity, thus reducing the formation period and increasing the initial cone tip diameter of the liquid cone. After the liquid bridge is formed, both the circuit current implying the liquid wetted area on the probe surface and the lifting velocity of the probe are utilized to further regulate the volume of the loaded droplet. Loaded droplets ranging from 60 to 600 pL are obtained via the method with a standard deviation of 4 to 30 pL. Moreover, a dot array is transferred with different loaded droplets. The minimum diameter of the printed dots is about 140 μm with a variation less than 5%. The advantages include the reduced risk of contamination, the droplet-size independent of the size of the probe, and the low cost of the device.

Durability and Recoverability of Soft Lithographically Patterned Hydrogel Molds for the Formation of Phase Separation Membranes

Hydrogel-facilitated phase separation (HFPS) has recently been applied to make microstructured porous membranes by modified phase separation processes. In HFPS, a soft lithographically patterned hydrogel mold is used as a water content source that initiates the phase separation process in membrane fabrication. However, after each membrane casting, the hydrogel content changes due to the diffusion of organic solvent into the hydrogel from the original membrane solution. The absorption of solvent into the hydrogel mold limits the continuous use of the mold in repeated membrane casts. In this study, we investigated a simple treatment process for hydrogel mold recovery, consisting of warm and cold treatment steps to provide solvent extraction without changing the hydrogel mold integrity. The best recovery result was 96%, which was obtained by placing the hydrogel in a warm water bath (50 °C) for 10 min followed by immersing in a cold bath (23 °C) for 4 min and finally 4 min drying in air. This recovery was attributed to nearly complete solvent extraction without any deformation of the hydrogel structure. The reusability of hydrogel can assist in the development of a continuous membrane fabrication process using HFPS.