Demonstration of Pattern Size Effects on Hydrophobic Nanocellulose Coatings with Regular Micron-Sized Island-like Geometrical Domains Created by Femtosecond Laser Micromachining

Abstract

As inspired by nature, wettability of bio-based material surfaces can be controlled by combining appropriate surface chemistries and topographies mimicking the structure of plant leaves or animals. The need for bio-based nanocellulose coatings with enhanced hydrophobic properties becomes technically relevant for extending their applications in the technological domain with better protection and lifetime of the coatings. In this work, the water repellence of spray-coated nanocellulose coatings with hydrophobically modified cellulose microfiber (mCMF coatings), or hydrophobically modified cellulose nanofiber (mCNF coatings) was enhanced after femtosecond laser patterning. In particular, the influences of different island-like pattern geometries and pattern sizes were systematically studied. The island-like patterns were experimentally created with single posts that have variable sizes of the valleys (B = 30 to 15 µm) and top surface area (T = 120 to 15 µm), resulting in good resolution of the patterns down to the size of the laser beam diameter (15 µm). Depending on the intrinsic homogeneity and porosity of sprayed mCMF and mCNF coatings, the quality and resolution of the island-like patterns is better for the mCNF coatings with thinner and more homogeneous sizes of the cellulose nanofibrils. The increase in apparent water contact angle on patterned nanocellulose coatings can be estimated from the theoretical Cassie-Baxter state of wetting and shows maximum values up to θ_s = 128° (mCMF coatings), or θ_s = 140° (mCNF coatings), for the smallest pattern sizes in parallel with minimum contact angle hysteresis of Δθ = 14° (mCMF coatings), or Δθ < 9° (mCNF coatings). The study demonstrated that femtosecond laser patterning technology provides high flexibility and adaptivity to create surface patterns in appropriate dimensions with enhanced hydrophobicity of nanocellulose coatings.

Keywords: dimensions; femtosecond laser; hydrophobicity; island-like geometry; nanocellulose coating; patterning.

Figure 1

Schematic representation of the laser beam path focusing on the sample set-up used for the femtosecond laser patterning.

Figure 2

Spatial overlap between single laser pulses as determined by the net effective pulse number η: (a) No pulse overlap, η < 1; (b) Pulse touching, η = 1; (c) Pulse overlap η > 1.

Figure 3

Schematic picture for creation of island-like geometrical domains with different sizes: (a) Representation of a single square post with dimensions H, B, T; (b) Path of the laser beam with spacing of the laser spot and overlap rate, with selection of hatch pitch hx, hy in respective X- and Y-direction; (c) Selection of smaller hatch pitch hx, hy in respective X- and Y-direction; (d) Selection of an offset value Δhx, Δhy for the hatch pitch during two subsequent laser ablation steps, (e) Selection of an offset value Δhx, Δhy for the hatch pitch during three subsequent laser ablation steps. The latter multiple laser processing steps are applied to increase B sizes and decrease T sizes.

Figure 4

Confocal laser microscopy of sprayed nanocellulose coatings before laser patterning: (a) Hydrophobic cellulose microfiber coatings (mCMF); (b) Hydrophobic cellulose nanofiber coatings (mCNF), with laser image (top) and 3D topographical image (bottom) in two magnifications (50×, 150× objective lens). The z-scale range applies to all topographical images.

Figure 5

Confocal laser interferometry of island-like domains on mCMF coatings created by femtosecond laser patterning with different geometrical dimensions of individual posts with selected T, B sizes (series according to Table 1).

Figure 6

Confocal laser interferometry of island-like domains on mCNF coatings created by femtosecond laser patterning with different geometrical dimensions of individual posts with selected T, B sizes (series according to Table 1).

Figure 7

Detailed microscopic observation of femtosecond laser-patterned mCMF coatings with an example of single island-like domains: (a) Local microscopy of island-like domains with sizes B = 15 µm, T = 50 µm (series 01), laser intensity image; (b) Long-range detail and island-like domains with sizes B = T = 20 µm (series 03), optical image; (c) Height profile over several posts on mCMF coatings.

Figure 8

Detailed microscopic observation of femtosecond laser-patterned mCNF coatings with an example of single island-like domains: (a) Local microscopy of island-like domains with sizes B = 15 µm, T = 50 µm (series 01), laser intensity image; (b) Long-range detail and island-like domains with sizes B = T = 20 µm (series 03), optical image; (c) Height profile over several posts on mCNF coatings.

Figure 9

Height dimensions (H) of individual posts on island-like geometrical patterns for mCMF coatings (black bar) and mCNF coatings (gray bar), as experimentally determined from profilometry measurements.

Figure 10

Three-dimensional topographical images of island-like domains on mCNF coatings created by femtosecond laser patterning with different geometrical dimensions of individual posts according to the selected T, B sizes (series according to Table 1). Same color applies for all Z scales.

Figure 11

Theoretical model calculations following the Cassie–Baxter equation of apparent contact angles θ* to be expected on mCMF coatings (open symbols, o) and mCNF coatings (closed symbols, ●), after island-like patterning with different H, B, T sizes of individual posts (see insets). The calculated values for θ* are plotted as a function of (a) H/B ratio, (b) B/T ratio, and (c) H/T ratio.

Figure 12

Hydrophobic properties of patterned mCMF coatings with island-like geometries of different B, T size (series according to Table 1): (a) Static water contact angles θ* (theoretical calculations in black bar, experimental value in gray bar); (b) Contact angle hysteresis of water Δθ (difference in advancing and receding contact angles).

Figure 13

Hydrophobic properties of patterned mCNF coatings with island-like geometries of different B, T size (series according to Table 1): (a) Static water contact angles θ* (theoretical calculations in black bar, experimental value in gray bar); (b) Contact angle hysteresis of water Δθ (difference in advancing and receding contact angles).

Figure 14

Comparative graph for values of apparent water contact angles, including theoretical values (Cassie–Baxter model) versus experimental values on patterned mCMF coatings (open symbols, o) and patterned mCNF coatings (closed symbols, ●).