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Electrospinning is used to fabricate microscale to nanoscale materials from polymeric solutions based on electrohydrodynamics. Material modifications are achieved through physical and chemical processes, producing diverse material architectures, from laboratory to industrial scales, for conventional and emerging applications. This Primer explains electrospinning technology, encompassing principles, methodologies, equipment, materials, applications, scalability and optimization. The article begins by elucidating the working principles, providing an overview of electrospinning methods and process parameters at laboratory and industrial scales, and discussing emerging equipment. Methods are described for tailoring the composition, architecture and properties of electrospun fibres and fibre assemblies. The versatility of these properties makes electrospun materials suitable for diverse applications spanning environmental, energy and medical applications, textiles, wearables, agriculture and advanced materials. The Primer concludes by discussing the constraints of current electrospinning techniques and offers a perspective on the field’s potential future trajectory.Ultra-fine nanofibres can be produced by manipulating polymer solutions, melts or suspensions with a strong electric field. This electrospinning process enables the properties of nanofibres to be tuned. In this Primer, electrospinning technology is explored, including the underlying principles, experimental techniques, characterization and applications in biomedicine, wearables and environmental purification.

Traditional fluorescence-based tags, used for anticounterfeiting, rely on primitive pattern matching and visual identification; additional covert security features such as fluorescent lifetime or pattern masking are advantageous if fraud is to be deterred. Herein, we present an electrohydrodynamically printed unicolour multi-fluorescent-lifetime security tag system composed of lifetime-tunable lead-halide perovskite nanocrystals that can be deciphered with both existing time-correlated single-photon counting fluorescence-lifetime imaging microscopy and a novel time-of-flight prototype. We find that unicolour or matching emission wavelength materials can be prepared through cation-engineering with the partial substitution of formamidinium for ethylenediammonium to generate “hollow” formamidinium lead bromide perovskite nanocrystals; these materials can be successfully printed into fluorescence-lifetime-encoded-quick-read tags that are protected from conventional readers. Furthermore, we also demonstrate that a portable, cost-effective time-of-flight fluorescence-lifetime imaging prototype can also decipher these codes. A single comprehensive approach combining these innovations may be eventually deployed to protect both producers and consumers. Designing effective covert security features is highly regarded to deter counterfeit of goods and currency in the global markets. Here, the authors present an electrohydrodynamically printed unicolour multifluorescent-lifetime security tag system based on perovskite to provide an alternative yet affordable solution.

The properties and structure of electrically stressed ionic liquid menisci experiencing ion evaporation are simulated using an electrohydrodynamic model with field-enhanced thermionic emission in steady state for an axially symmetric geometry. Solutions are explored as a function of the external background field, meniscus dimension, hydraulic impedance and liquid temperature. Statically stable solutions for emitting menisci are found to be constrained to a set of conditions: a minimum hydraulic impedance, a maximum current output and a narrow range of background fields that maximizes at menisci sizes of 0.5–3 ${\\rm \\mu}{\\rm m}$ in radius. Static stability is lost when the electric field adjacent to the electrode that holds the meniscus corresponds to an electric pressure that exceeds twice the surface tension stress of a sphere of the same size as the meniscus. Preliminary investigations suggest this limit to be universal, therefore, independent of most ionic liquid properties, reservoir pressure, hydraulic impedance or temperature and could explain the experimentally observed bifurcation of a steady ion source into two or more emission sites. Ohmic heating near the emission region increases the liquid temperature, which is found to be important to accurately describe stability boundaries. Temperature increase does not affect the current output when the hydraulic impedance is constant. This phenomenon is thought to be due to an improved interface charge relaxation enhanced by the higher electrical conductivity. Dissipated ohmic energy is mostly conducted to the electrode wall. The higher thermal diffusivity of the wall versus the liquid, allows the ion source to run in steady state without heating.

We investigate the linear stability threshold of a dielectric liquid subjected to unipolar injection in a two-dimensional rectangular enclosure with rigid boundaries. A finite element formulation transforms the set of linear partial differential equations that governs the system into a set of algebraic equations. The resulting system poses an eigenvalue problem. We calculate the linear stability threshold, as well as the velocity field and charge density distribution, as a function of the aspect ratio of the domain. The stability parameter as a function of the aspect ratio describes paths of symmetry-breaking bifurcation. The symmetry properties of the different linear modes determine whether these paths cross each other or not. The resulting structure has important consequences in the nonlinear behaviour of the system after the bifurcation points.

Ceramic aerogels are attractive for many applications due to their ultralow density, high porosity, and multifunctionality but are limited by the typical trade-off relationship between mechanical properties and thermal stability when used in extreme environments. In this work, we design and synthesize ceramic nanofibrous aerogels with three-dimensional (3D) interwoven crimped-nanofibre structures that endow the aerogels with superior mechanical performances and high thermal stability. These ceramic aerogels are synthesized by a direct and facile route, 3D reaction electrospinning. They display robust structural stability with structure-derived mechanical ultra-stretchability up to 100% tensile strain and superior restoring capacity up to 40% tensile strain, 95% bending strain and 60% compressive strain, high thermal stability from −196 to 1400 °C, repeatable stretchability at working temperatures up to 1300 °C, and a low thermal conductivity of 0.0228 W m −1 K −1 in air. This work would enable the innovative design of high-performance ceramic aerogels for various applications. Ceramic aerogels are generally brittle and often tend to structurally collapse under large external tensile strain. Here the authors synthesize large-scale stretchable ceramic aerogels with interwoven crimped nanofibers by combining electrohydrodynamic method and 3D reaction electrospinning.

An isolated charge-neutral droplet in a uniform electric field experiences no net force. However, a droplet pair can move in response to field-induced dipolar and hydrodynamic interactions. If the droplets are identical, the centre of mass of the pair remains fixed. Here, we show that if the droplets have different properties, the pair experiences a net motion due to non-reciprocal electrohydrodynamic interactions. We analyse the three-dimensional droplet trajectories using asymptotic theory, assuming spherical droplets and large separations, and numerical simulations based on a boundary integral method. The dynamics can be quite intricate depending on the initial orientation of the droplets line-of-centres relative to the applied field direction. Drops tend to migrate towards a configuration with line-of-centres either parallel or perpendicular to the applied field direction, while either coming into contact or indefinitely separating. We elucidate the conditions under which these different interaction scenarios take place. Intriguingly, we find that in some cases droplets can form a pair (tandem) that translates either parallel or perpendicular to the applied field direction.

Weakly conducting dielectric liquid drops suspended in another dielectric liquid and subject to an applied uniform electric field exhibit a wide range of dynamical behaviours contingent on field strength and material properties. These phenomena are best described by the Melcher–Taylor leaky dielectric model, which hypothesizes charge accumulation on the drop–fluid interface and prescribes a balance between charge relaxation, the jump in ohmic currents from the bulk and charge convection by the interfacial fluid flow. Most previous numerical simulations based on this model have either neglected interfacial charge convection or restricted themselves to axisymmetric drops. In this work, we develop a three-dimensional boundary element method for the complete leaky dielectric model to systematically study the deformation and dynamics of liquid drops in electric fields. The inclusion of charge convection in our simulations permits us to investigate drops in the Quincke regime, in which experiments have demonstrated a symmetry-breaking bifurcation leading to steady electrorotation. Our simulation results show excellent agreement with existing experimental data and small-deformation theories.

When a droplet coalesces with a flat liquid–air interface, a secondary drop may be left behind resulting in only a partial coalescence rather than complete coalescence. In this paper, we employ an arbitrary Lagrangian–Eulerian method to demonstrate that applying an electric field favours the occurrence of partial coalescence. To understand this phenomenon, we systematically study the effect of an external electric field on the coalescence process between a droplet and a liquid–air interface. In an electric field, the induced electric stresses can overcome the downward flow of the droplet, thus lifting it upwards. As a result, the positive Laplace pressure in the neck region squeezes the droplet towards pinch-off. We observe that both the initial neck expansion and neck shrinkage are suppressed by the electric field. These effects become weaker as the Ohnesorge number $Oh$ increases. Based on the scaling analysis, we report a critical Ohnesorge number $O{h_c} = 14.39{\\varGamma ^{3/2}} + 0.029$ to quantify the transition from partial coalescence to complete coalescence in the presence of an electric field, where $\\varGamma $ represents the dimensionless electric Bond number. Finally, a relationship between the secondary droplet size and the two key dimensionless numbers of $Oh$ and $\\varGamma $ has been developed, which could be useful for producing droplets of desired sizes in microfluidic applications.

Additive manufacturing technologies based on layer-by-layer deposition of material ejected from a nozzle provide unmatched versatility but are limited in terms of printing speed and resolution. Electrohydrodynamic jetting uniquely allows generating submicrometer jets that can reach speeds above 1 m s −1 , but such jets cannot be precisely collected by too slow mechanical stages. Here, we demonstrate that controlling the voltage applied to electrodes located around the jet, its trajectory can be continuously adjusted with lateral accelerations up to 10 6  m s −2 . Through electrostatically deflecting the jet, 3D objects with submicrometer features can be printed by stacking nanofibers on top of each other at layer-by-layer frequencies as high as 2000 Hz. The fast jet speed and large layer-by-layer frequencies achieved translate into printing speeds up to 0.5 m s −1 in-plane and 0.4 mm s −1 in the vertical direction, three to four orders of magnitude faster than techniques providing equivalent feature sizes. Additive manufacturing technologies provide versatility but are limited in terms of printing speed and resolution. Here the authors demonstrate printing of 3D structures with submicrometer features by electrostatically deflecting a jet in electrohydrodynamic jetting.