Ordering in Dewetting of a Thin Polymer Bilayer with a Topographically Patterned Interface

Abstract

We report that dewetting of a thin polymer bilayer consisting of a stacked polystyrene (PS) film on top of a topographically patterned poly(methyl methacrylate) (PMMA) bottom layer on a nonwettable substrate results in a myriad of partially or fully ordered patterns. The local thicknesses of the thinnest parts of both the layers, that is, the residual layer thickness of the PMMA film after patterning (h R-PMMA ) and the local thickness of the PS film over the PMMA stripes (h T-PS ), collectively influence the evolution pathway and the extent of the ordering of the final dewetted features. In case both h R-PMMA and h T-PS are very low (≤9 nm), the films undergo near-simultaneous rupture over their respective thinnest areas, resulting in a morphology consisting of an array of ordered PS droplets surrounded by a holey PMMA matrix. In contrast, slightly thicker h T-PS (≈12 nm) leads to sequential rupture of the PMMA and the PS films over the substrate grooves, resulting in an array of core (PMMA)−shell (PS) threads. While the top layer flattens out along with the suppression of instability with a further increase in h T-PS (≈22 nm), the bottom PMMA layer still undergoes submerged directed dewetting. On the other hand, a slight increase in h R-PMMA ≥ 13 nm suppresses pattern-directed rupture of the bottom layer due to surface-tension-mediated flattening. Even on such a thicker bottom layer, an ordered array of aligned PS droplets exhibiting a Neumann configuration is obtained when h T-PS ≈ 9 nm. With a further increase in h T-PS , the ordering is completely lost in both layers. The nonwettability of the substrate hinders late-stage lateral coarsening of the dewetting features, facilitating the formation of ordered meso patterns by dewetting of a thin polymer bilayer with a topographically patterned interface. Finally, we also explore how various parameters such as the height of the PMMA stripes (h S-PMMA ), periodicity of the patterns (λ S ), and viscosity of the two layers influence the final dewetted features.

Non-wetting Liquid-Infused Slippery Paper

“…It is well known that if a continuous film is deposited over a topographically patterned surface, the top surface of the film has an undulating appearance, and the local film thickness gets lower over the protrusions of the substrate features. 65 Consequently, there are periodic domains hallmarking high and low film thickness region, which result in alternating domains of low and high slip length, which is evident from the theoretical model predictions depicted in Figure 5 earlier. The lower slip velocity regions imparted resistance to the flow of the liquid over the protrusions as shown in Figure 7(A2).…”

“…Due to the corrugation, the surface under the droplet in the direction transverse to the aligned structures encountered periodically varying depth of the oil layer, as schematically shown in Figure (A1). It is well known that if a continuous film is deposited over a topographically patterned surface, the top surface of the film has an undulating appearance, and the local film thickness gets lower over the protrusions of the substrate features . Consequently, there are periodic domains hallmarking high and low film thickness region, which result in alternating domains of low and high slip length, which is evident from the theoretical model predictions depicted in Figure earlier.…”

Nanometer-Thick Superhydrophobic Coating Renders Cloth Mask Potentially Effective against Aerosol-Driven Infections

“…Moreover, the conformal topography formed by the thin layer of coated OTS over the textured silk fabric made the ellipsometric measurements difficult over the cloth. 54 Instead, cleaned silicon wafer was ozonetreated for 30 min followed by a 30 min OTS treatment and then measured with an ellipsometer for thickness as an alternative. The thickness of the OTS layer was measured to be 1.9 ± 0.1 nm with a root-mean-square error (RMSE) of 0.585 (the operating data used in the ellipsometer are provided in the Supporting information, SI).…”

“…As laser light interacts with cloth fabric, thickness measurement on cloth was not feasible. Moreover, the conformal topography formed by the thin layer of coated OTS over the textured silk fabric made the ellipsometric measurements difficult over the cloth . Instead, cleaned silicon wafer was ozone-treated for 30 min followed by a 30 min OTS treatment and then measured with an ellipsometer for thickness as an alternative.…”

Nanoparticle-Mediated Stabilization of a Thin Polymer Bilayer

“…In contrast to the array of hemispherical droplets, which is invariably obtained in dewetting of a single-layer film, a variety of exotic and novel structures such as submerged, embedded, encapsulated, hollow, or core–shell type droplets have been obtained because of dewetting of a thin polymer bilayer. Furthermore, the inherently random and isotropic dewetted features can be aligned into well-ordered periodic structures by dewetting the bilayer on a topographically patterned substrate or when the bilayer has a topographically patterned interface . In another class of problem, the bottom layer does not participate in dewetting, and consequently, the problem reduces to dewetting of a liquid thin film on a stable liquid surface, resulting in final dewetted droplets that exhibit Neumann configuration. − In such cases, depending on the viscosity of the bottom layer, the rims surrounding the dewetted holes in the top layer may penetrate the bottom layer (when bottom layer viscosity is low), thereby imparting additional pinning resistance against hole growth, , or the top film can exhibit rapid dewetting because of enhanced interfacial slippage (when bottom layer viscosity is high) − and result in noncircular faceted features …”