Electrohydrodynamic Atomization

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1		Chengbo Ru	--	1362	2022-04-12 10:23:08	\|
2	format correct	Catherine Yang	-13 word(s)	1349	2022-04-12 10:51:30	\|

This entry is adapted from the peer-reviewed paper 10.3390/molecules27072374

Electrohydrodynamic atomization, which comprises electrospray and electrospinning, is a physical process induced by an external electric field. The setup of EHDA is relatively simple, consisting of a high-voltage source (positive or negative) that provides external electrical energy and a precursor feeding source mounted with a metal capillary nozzle facing a conductive collector. Although EHDA can be operated in an atmospheric environment, the setup is usually placed in a fume hood with good electrical isolation and ventilation to ensure safety while maintaining the temperature and relative humidity of the workspace.

electrospray electrospinning energetic material

1. Process of Electrohydrodynamic Atomization

When the precursor flows through the metal capillary nozzle, the competition of the external electric force and surface tension deforms the meniscus. At a low voltage, the external electric force is too weak to overcome the surface tension, and then the precursor flows dropwise, which is known as the dripping mode. When the applied voltage is increased, a stronger external electrical force causes the meniscus to elongate into a cone shape known as the Taylor cone. Then, a fine jet (much finer than the nozzle) is ejected from the tip of the Taylor cone; this phenomenon is called the cone-jet mode and is often used due to its repeatability and controllability.

1.1. Electrospray

When Newtonian liquids or dilute viscoelastic liquids are used, the emitted jet breaks into fine, charged droplets due to the instability caused by the accumulated surface charge, termed electrospray. The diameter of the charged droplets (d_d) can be estimated by Equation (1) ^[1]:

（1）

where α is a constant, Q is the solution flow rate, ρ is the solution density, γ is the solution conductivity, σ is the liquid surface tension, and ε0 is the electric permittivity of the free space. Then, these charged droplets fly to the collector, driven by electrostatic force and accompanied by solvent evaporation. Due to solvent evaporation, the droplets shrink, and then the surface charge density increases. When the surface charges of the droplets accumulate and reach the Rayleigh limit, electrostatic repulsion and Coulomb fission occur, forming smaller droplets. This threshold of surface charge q_R can be estimated by following equation ^[2]:

（2）

where R is the droplet radius. If the solvent in the charged droplets completely evaporates before landing on the substrate, then dry particles with a narrow size distribution can be collected. After evaporation, the remained diameter (d_p) of solid particles can be deduced by mass conservation ^[1]^[3]

(3)

where ω is the weight fraction of solid material, ρs is the density of the solvent and ρp is the density of solid material. When solvent evaporation is insufficient, wet or semidry particles coat the surface of the collector, forming a thin film. The drying degree of the deposited particles can be tuned by choosing solvents with different saturated vapor pressures.

1.2. Electrospinning

When viscoelastic precursors with dissolved polymers are used, the emitted jet experiences bending instability driven by an external electrical field and then elongates into filament due to enough polymer chain entanglements; this process is known as electrospinning. Based on volume conservation, the diameter of electrospun fibers d_j can be predicted by the following equation ^[4] without considerations of elastic effects and solvent evaporation:

(4)

where, ε is the dielectric constant of the outside medium, I is the electric current through the jet, and χ is the dimensionless wavelength of the bending instability. Then, solidified fibers with diameters down to the nanoscale deposit on the collector and form a nonwoven mat.

2. Transition between Electrospinning and Electrospray

The basic principles and setups of electrospray and electrospinning are similar, the only difference being the travel motion of the emitted jet in the external electrical field. Upon the addition of polymers into the precursors, a transition between electrospinning and electrospray may occur, dominated by the various rheological properties of the precursor, such as viscosity, surface tension and chain entanglement behaviors, which are mainly determined by the molecular weight (M_w) and concentration of the polymer.

The precursors can be classified into the dilute regime, the semidilute unentangled regime, the semidilute entangled regime, and the concentrated regime in terms of their polymer concentrations ^[5]. The jets emitted under these regimes show distinct dynamic behaviors and produce different structures of particles, beaded fibers and uniform fibers under identical operation parameters. In the dilute regime, no overlap of polymer chains occurs. In the semidilute unentangled regime, some polymer chains overlap, but they do not overlap enough to entangle each other. Then, without topological constraints or chain entanglements, the emitted jet breaks into droplets induced by external electrical force and surface tension, namely electrospray. In the semidilute entangled regime, obvious polymer chain entanglements dominate the deformation of the emitted jet, and discontinuous beaded fibers can be produced, in which short nanofibers connect large particles. As the concentration increases, the shape of the beads changes from spherical to elongated and then spindle like, ultimately disappearing, forming continuous fibers in the concentrated regime. The threshold between these regimes can be expressed as the critical chain overlap concentration co (boundary of the dilute regime and the semidilute unentangled regime) and the critical entanglement concentration ce (boundary of the semidilute unentangled regime and the semidilute entangled regime), which can be measured by exploring the dependence of the specific viscosity on the concentration ^[6].

The molecular weight (M_w) decides the chain length and occupied hydrodynamic volume of the applied polymer and further affects the rheological behaviors and chain overlap behaviors. Polymers with higher Mw possess a higher hydrodynamic volume and consequently have a lower c_o and c_e. For poly(methyl methacrylate) (PMMA) in N,N-dimethylformamide (DMF), the values of c_o are 10.2 wt% and 2.5 wt% when the M_w values are 12,470 g·mol⁻¹ and 205,800 g·mol⁻¹, respectively ^[6].

3. Morphology

As expressed by the aforementioned scaling laws, the size distribution and morphology of the prepared materials can be easily regulated by various parameters, which can be classified as precursor parameters (electrical conductivity, surface tension, viscosity, volatility, concentration, and the molecular weight of polymer), and operation parameters (electric field strength, flow rate, and distance from the needle to collector) ^[1]^[7]^[8]. In addition, ambient parameters ^[9] (temperature, and relative humidity) may affect the evaporation kinetics and viscosity of precursor, and afterwards influence the process of EHDA. Then, bulk materials can be processed into particles, fibers, films and 3D structures in a “bottom-to-up” manner. Unlike the ultrasonic mixing of composites occurring in a macroscopic system, EHDA is a physical recrystallization or mixing process which may involves a chemical reaction in confined tiny droplets or elongated jets. A benefit of this procedure is that tiny crystals and composite materials with homogeneously dispersed components can be obtained. The feature size of the obtained materials usually falls within the micron to nanometer range with monodispersity. Fast solvent evaporation leads to pore formation on the surface and the interior of the assembled particles as insufficient time to rearrange polymer chains or solutes, resulting in a porous morphology. In contrast, solvents with a low vapor pressure lead to the formation of smooth morphology. Moreover, particles and fibers with core–shell structures or coaxial structures can be readily obtained by a coaxial needle.

Electrosprayed dry particles are generally wrinkled, hollow spheres, sponge-like spheres or compact spheres; their morphologies are determined by solvent evaporation, the rigidity of the elastic shell, the mechanical balance of inner and ambient pressures, surface tension stress and electrical normal stresses ^[8]. In the case of film deposition, the electrosprayed droplets must remain wet when landing on the substrate so that the polymer chains or solutes can rearrange before solidification. Therefore, the prepared films are generally dense and have no obvious cracks without further thermal treatment ^[10]. The electrospun nonwoven mats are composed of fibers with inter-/intrafibrous porosity. After elongation, the diameter of electrospun fibers can be reduced to a few tens of nanometers. With a small diameter, the macropores between the fibers and the mesopores or micropores on the surface of the fibers also contribute to a high specific surface area. This unique structure gives the nonwoven textile specific functionalities, such as filtration, cell seeding and attachment, substance transport and exchange. With these excellent morphologies and structures, EMs can show superior reactivity in the forms of particles (0D), fabric mats composed of nanofibers (1D) and films (2D).

References

Jaworek, A.; Sobczyk, A.T.; Krupa, A. Electrospray application to powder production and surface coating. J. Aerosol Sci. 2018, 125, 57–92.
Rayleigh, L. On the equilibrium of liquid conducting masses charged with electricity. Lond. Edinb. Dublin Philos. Mag. J. Sci. 1882, 14, 184–186.
Xie, J.; Jiang, J.; Davoodi, P.; Srinivasan, M.P.; Wang, C.-H. Electrohydrodynamic atomization: A two-decade effort to produce and process micro-/nanoparticulate materials. Chem. Eng. Sci. 2015, 125, 32–57.
Fridrikh, S.V.; Yu, J.H.; Brenner, M.P.; Rutledge, G.C. Controlling the fiber diameter during electrospinning. Phys. Rev. Lett. 2003, 90, 144502–144505.
McKee, M.G.; Wilkes, G.L.; Colby, R.H.; Long, T.E. Correlations of Solution Rheology with Electrospun Fiber Formation of Linear and Branched Polyesters. Macromolecules 2004, 37, 1760–1767.
Gupta, P.; Elkins, C.; Long, T.E.; Wilkes, G.L. Electrospinning of linear homopolymers of poly(methyl methacrylate): Exploring relationships between fiber formation, viscosity, molecular weight and concentration in a good solvent. Polymer 2005, 46, 4799–4810.
Xue, J.; Wu, T.; Dai, Y.; Xia, Y. Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications. Chem. Rev. 2019, 119, 5298–5415.
Bodnar, E.; Grifoll, J.; Rosell-Llompart, J. Polymer solution electrospraying: A tool for engineering particles and films with controlled morphology. J. Aerosol Sci. 2018, 125, 93–118.
Greiner, A.; Wendorff, J.H. Electrospinning: A Fascinating Method for the Preparation of Ultrathin Fibers. Angew. Chem. Int. Ed. 2007, 46, 5670–5703.
Kelder, E.M.; Marijnissen, J.C.M.; Karuga, S.W. EDHA for energy production, storage and conversion devices. J. Aerosol Sci. 2018, 125, 119–147.

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Chengbo Ru

Gang Li

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