Modeling resistance of nanofibrous superhydrophobic coatings to hydrostatic pressures: The role of microstructure

Abstract

A structural viscosity model for magnetorheology Appl. Phys. Lett. 101, 021903 (2012) Self-propulsion in viscoelastic fluids: Pushers vs. pullers Phys. Fluids 24, 051902 (2012) Two regimes of self-propelled motion of a torus rotating about its centerline in a viscous incompressible fluid at intermediate Reynolds numbers Phys. Fluids 24, 053603 (2012) Helical flows of fractionalized Burgers' fluids AIP Advances 2, 012167 (2012) Additional information on Phys. FluidsIn this paper, we present a numerical study devised to investigate the influence of microstructural parameters on the performance of fibrous superhydrophobic coatings manufactured via dc and ac electrospinning. In particular, our study is focused on predicting the resistance of such coatings against elevated hydrostatic pressures, which is of crucial importance for submersible applications. In our study, we generate 3D virtual geometries composed of randomly or orthogonally oriented horizontal fibers with bimodal diameter distributions resembling the microstructure of our electrospun coatings. These virtual geometries are then used as the computational domain for performing full morphology numerical simulations to establish a relationship between the coatings' critical pressure (pressure beyond which the surface may depart from the Cassie state) and their microstructures. For coatings with ordered microstructures, we have also derived analytical expressions for the critical pressure based on the balance of forces acting on the water-air interface. Predictions of our force balance analysis are compared with those of our FM simulations as well as the equations proposed by Tuteja et al. [Proc. Natl. Acad. Sci. U.S.A. 105, 18200 (2008)], and discussed in detail. Our numerical simulations are aimed at providing useful information with regards to the tolerance of fibrous superhydrophobic coatings against elevated pressures, and helping with the design and optimization of the coatings' microstructures. Our results show considerably higher pressure tolerance for the case of coatings with orthogonally oriented fibers as compared to those with randomly laid fibers when other microstructural parameters are held constant. Moreover, it is demonstrated that thickness of the coating has less influence on performance in the case of orthogonal microstructures. Coatings' responses to other variations favor those that yield smallersized inter-fiber spaces. Studies are also performed investigating the effect of subtle permutations in the layer configurations of our ac-electrospun coatings, as well as the use of a hybrid coating that utilizes advantages from both dc and ac electrospinning. C 2012 American Institute of Physics. [http://dx.

Polymeric Slippery Coatings: Nature and Applications

“…The deflection of the meniscus was calculated at a hydrostatic pressure of 3400 Pa. In a related numerical study, Bucher et al [96] conducted a series of FM simulations to study the stability of an air-water interface formed over superhydrophobic coatings composed of randomly oriented nanofibers. The study was achieved for 3-D virtual domains resembling the internal microstructure of electrospun fiber mats.…”

Wetting resistance of heterogeneous superhydrophobic coatings with orthogonally layered fibers

“…(3) and (5) for the simple case of two parallel fibers, and established a range of contact angles and fiber spacing where the two approaches closely agree [40].…”

“…1, the capillary resistance is stronger, and the solid-water contact area is different for each fiber-layer with which the AWI is in contact. This is compounded by another issue: for a single set of parallel fibers, complete wetting takes place when the AWI penetrates deep enough to reach a critical profile, at a critical immersion angle of α cr , where the balance of mechanical forces over the AWI is no longer maintained [40,41]. As will be shown later in Section 3, our study indicates that an AWI that straddles across two or more layers of orthogonal fibers most often does not reach such a mechanical breaking point before it deflects laterally so as to meet itself under the fibers (across the symmetry boundaries).…”

“…The only exception to this is the case of thin coatings with the air under the AWI completely trapped, like when a coating is fully submerged (in this case the entrapped air significantly contributes to the coating's resistance against the hydrostatic pressure [38,39]). For thick coatings (i.e., coatings for which the volume of the air displaced by a penetrating AWI is negligible compared to the volume of the air in the coating), even when submerged, the capillary pressure is the dominant force balancing the hydrostatic pressure exerted on the coating [40].…”

“…We previously used the Full Morphology (FM) algorithm to predict the resistance of submerged SHP fibrous coatings under elevated hydrostatic pressures [40]. The underlying principle of the FM method is to relate the size of the largest sphere that can fit into the void space between the fibers at any point in the 3-D domain to a fictitious pressure obtained from the Young-Laplace equation written for a capillary tube [42,43]:…”