Self-limiting Electrospray Deposition Research Articles

Sub‐Micron Thickness Self‐Limiting Electrospray Deposition via Voltage Bias of Spray Target

AbstractElectrospray deposition (ESD) is a technique with emerging relevance for applications requiring micro‐/nano‐scaled coatings. Self‐limiting electrospray deposition (SLED) is a regime of ESD where upon satisfying the criteria for self‐limiting behavior, the charge buildup of the material will eventually begin to repel itself until an asymptotic thickness is achieved, based primarily on the properties of the spray solution and deposited material. This work focuses on controlling the coating thickness further to achieve sub‐micron thicknesses. By applying a voltage bias of the same polarity of the spray onto the target substrate while providing a secondary grounded surface, the thickness of a payload can be manipulated for thinner and more precise thicknesses. The magnitude of the voltage bias is varied to show the effect of voltage bias on thickness, and 2‐D and 3‐D structures are used to demonstrate the generality of this technique for complex substrates. The results show how SLED can be included as a potential alternative for conformal coatings deposited at ambient conditions at the sub‐micron scale.

Non‐destructive thickness measurement of optically scattering polymer films using image processing

AbstractWe establish a sample‐ and data‐processing pipeline that allows for high‐throughput optical microscope measurement of porous films, provided they are sufficiently optically scattering. Here, self‐limiting electrospray deposition (SLED) is used to manufacture scattering films of different morphologies. This technique compensates for the scattering of the films through background subtraction of the reflection image with the transmission image. This process is implemented through a combination of an ImageJ and MATLAB data pipeline; the Canny edge‐detector is used as the image‐processing algorithm to identify the boundaries of the film. This process is verified against manually measured images; a comparative study between cross‐sectional scanning electron microscopy (where scattering effects are diminished) and optical microscopy also verifies that our optical microscopy technique can be used to consistently, non‐destructively measure film thickness regardless of film morphology. In addition, this technique can be used in combination with dense film measurements to measure film porosity.

Microscale Templating of Materials across Electrospray Deposition Regimes

Electrospray deposition (ESD) uses strong electric fields to produce generations of monodisperse droplets from solutions and dispersions that are driven toward grounded substrates. When soft materials are delivered, the behavior of the growing film depends on the film’s ability to dissipate charge, which is strongly tied to its mobility for dielectric materials. Accordingly, there exist three regimes of electrospray: electrowetting, charged melt, and self-limiting. In the self-limiting regime, it has been recently shown that the targeted nature of these sprays allows for corona-free 3D coating. While ESD patterning on the micron-scale has been studied for decades, most typically through the use of insulating masks, there has been no comparative study of this phenomenon across spray regimes. Here, we used test-patterns composed of gratings that range in both feature size (30–240 μm) and spacing (⅓x–9x) to compare materials across regimes. The sprayed patterns were scanned using a profilometer, and the density, average height, and specificity were extracted. From these results, it was demonstrated that material deposited in the self-limiting regime showed the highest uniformity and specificity on small features as compared to electrowetting and charged melt sprays. Self-limiting electrospray deposition is, therefore, the best suited for modification of prefabricated electrode patterns.

Self-limiting electrospray deposition on polymer templates

Electrospray deposition (ESD) applies a high voltage to liquids flowing through narrow capillaries to produce monodisperse generations of droplets down to hundreds of nanometers in diameter, each carrying a small amount of the delivered solute. This deposition method has been combined with insulated stencil masks for fabricating micropatterns by spraying solutions containing nanoparticles, polymers, or biomaterials. To optimize the fabrication process for micro-coatings, a self-limiting electrospray deposition (SLED) method has recently been developed. Here, we combine SLED with a pre-existing patterned polymer film to study SLED’s fundamental behavior in a bilayer geometry. SLED has been observed when glassy insulating materials are sprayed onto conductive substrates, where a thickness-limited film forms as charge accumulates and repels the arrival of additional charged droplets. In this study, polystyrene (PS), Parylene C, and SU-8 thin films of varying thickness on silicon are utilized as insulated spraying substrates. Polyvinylpyrrolidone (PVP), a thermoplastic polymer is sprayed below its glass transition temperature (Tg) to investigate the SLED behavior on the pre-deposited insulating films. Furthermore, to examine the effects of in-plane confinement on the spray, a microhole array patterned onto the PS thin film by laser dewetting was sprayed with dyed PVP in the SLED mode. This was then extended to an unmasked electrode array showing that masked SLED and laser dewetting could be used to target microscale regions of conventionally-patterned electronics.

Self-Limiting Electrospray Deposition for the Surface Modification of Additively Manufactured Parts.

Electrospray deposition (ESD) is a spray coating process that utilizes a high voltage to atomize a flowing solution into charged microdroplets. These self-repulsive droplets evaporate as they travel to a target substrate, depositing the solution solids. Our previous research investigated the conditions necessary to minimize charge dissipation and deposit a thickness-limited film that grows in area over time through self-limiting electrospray deposition. Such sprays possess the ability to conformally coat complex three-dimensional (3D) objects without changing the location of the spray needle or orientation of the object. This makes them ideally suited for the postprocessing of materials fabricated through additive manufacturing (AM), opening a paradigm of independent bulk and surface functionality. Having demonstrated 3D coating with film thickness in the range of 1-50 μm on a variety of conductive objects, in this study, we employed model substrates to quantitatively study the technique's limits with regard to geometry and scale. Specifically, we examined the effectiveness of thickness-limited ESD for coating recessed features with gaps ranging from 50 μm to 1 cm, as well as the ability to coat surfaces hidden from the line-of-sight of the spray needle. This was then extended to the coating of hydrogel structures printed by AM, demonstrating that coating could be conducted even into the body of the structures as a means to create hydrophobic surfaces without affecting the absorption-driven humidity response. Further, these coatings were robust enough to create superhydrophobicity in the entire structure, causing it to resist immersion in water.