SEM images of electrospun nanofibrous coatings fabricated using: (a) dc electrospinning, (b) biased-ac electrospinning. Our ability to produce these structures is the basis for our numerical study.10
Samples of our virtual, three dimensional, bimodal fibrous structures used in our numerical study, composed either of: (a) layered, randomly oriented fibers like those produced via dc electrospinning (SVF = 10%, df = 100 nm, Rcf = 3, nc = 0.1, t = 9.6 μm), or (b) layered, orthogonally oriented fibers like those produced via the biased ac method (SVF = 11.2%, df = 100 nm, Rcf = 3, nc = 0.1, t = 9.6 μm).
A visualization of different stages of water intrusion simulation into a unimodal fibrous structure with SVF of 10%, unimodal fiber diameter of 500 nm with random in-plane fiber orientation, using the full morphology method. Corresponding pressures are: (a) 58.983 kPa, (b) 68.235 kPa, (c) 77.333 kPa, and (d) 80.930 kPa. The non-wetting fluid (water) represented in the lighter-shaded region (red online) is made up of spheres fitted into the domain.
A visualization depicting the conceptual characterization of critical pressure for the case of ordered parallel fibers using the: (a) force balance and (b) FM approaches. The value of α* associated with the force balance method is plotted (c) as a function of SVF for different contact angles.
(a) Critical pressure as a function of SVF using four different methods: our force balance (FB), full morphology (FM), and the robustness angle and robustness height method of (Ref. 20) Eqs. (5)–(8), respectively. (b) The ratios , , and as a function of SVF. (c) The ratios of , , and as a function of contact angle. (d–f) Surface contour plots of , , and versus SVF and contact angle.
Capillary pressure–saturation curves for randomly oriented, layered structures. (a) and (b) vary only in the size of their domain (cubes of side length s); (c) and (d) vary only in their voxel resolution. Domain-size independence is acknowledged when breakthrough pressure, magnified for clarity in (b), no longer varies appreciably from one domain size to the next, taken as being greater than 10 μm. Voxel-size independence is acknowledged when critical pressure, magnified for clarity in (d), no longer varies appreciably from one voxel resolution to the next, taken as being when one voxel length is less than 0.33 d f .
Conceptual illustration of different stages of water penetration into a coating surface, the dark region (red online) representing the intruding water front: (a) water has not yet fully penetrated into the first layer, (b) interface has reached the second layer, but has not yet submerged the first (critical pressure is the maximum pressure value for this condition), and (c) coating failure has occurred; the first layer of fibers is fully submerged.
Capillary pressure–saturation curves for bimodal fibrous coatings of varying thickness composed of: (a) orthogonally oriented fibers (SVF = 11.2%, df = 100 nm, Rcf = 3, nc = 0.1), (b) randomly oriented fibers (SVF = 10%, df = 100 nm, Rcf = 3, nc = 0.1). The dotted cross through each plot is to better illustrate the p c value taken as the critical pressure for the respective coating type, once thickness independence has been established. Shaded circles in (b) correspond to critical pressure determined for coatings not yet thickness-independent.
Critical pressure predictions for layered, randomly oriented (dc-electrospun) media compared against variations in one of four microstructural parameters: (a) SVF, (b) fiber diameter (holding the diameter ratio between the two fiber sizes constant), (c) coarse-to-fine fiber diameter ratio (holding fine fiber diameter constant), and (d) coarse-fiber number fraction.
Example of capillary pressure–saturation curves for layered, orthogonally oriented structures with SVF = 11.2%, df = 100 nm, Rcf = 5, and nc = 0.1. The structures vary only in the magnitude of each fiber's departure from “perfectly ordered,” even spacing within a layer (Eq. (1)).
Critical pressure predictions for layered, orthogonally oriented (biased-ac-electrospun) media compared against variations in one of four microstructural parameters: (a) SVF, (b) fiber diameter (holding the diameter ratio between the two fiber sizes constant), (c) coarse-to-fine fiber diameter ratio (holding fine fiber diameter constant), and (d) coarse-fiber number fraction.
(a) An image of a hybrid coating, consisting of a 7.8-μm-thick layer of anisotropic-orthogonal fibers, and a 16.2-μm-thick layer of randomly oriented fibers, and (b) its capillary pressure–saturation curve, illustrating the difference in coating behavior depending on which layer is used as the top layer. The dashed crosses correspond to the critical pressure for each curve.
Default coating microstructure properties used in our parameter study. Parameters not being varied for study will correspond to this table.
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