Surfaces with alternating (super)hydrophilic and (super)hydrophobic areas can provide new opportunities to control the wetting behavior of surfaces in microfluidics applications, biomedical devices, and chemical synthesis, etc. The development of surfaces with special wettability can be achieved through artificial structuration, often in combination with chemical heterogeneity. Various biological materials are actually designed by nature to exhibit such surface properties. As an example, the desert beetle back are covered by bumps with no wax covering on peak for hydrophilicity while the troughs consist of microstructure coated in wax for hydrophobicity . As opposed to natural processes, in most cases people have to utilize costly and sophisticated fabrication technologies to generate artificial surfaces with special wettability patterns. Inspired by the peculiar ordered porous structure of wood at the microscale, we worked on a new approach to make direct use of the natural scaffold provided by spruce wood (Picea abies).
We show that the earlywood (EW) and latewood (LW) alternation (known as year rings) on a wood cross section can be used to design a wood-based material with anisotropic wetting properties, through a selective modification dictated by wood itself. EW and LW essentially differ due to their cell wall thicknesses, lumen sizes and component ratios etc. This leads to the differences in mechanical properties, which have in turn a strong influence on the wood cross section surface after the cutting process. By using circular saw cutting, we were able to generate a high roughness contrast between EW and LW regions: EW regions showed a rather open and rough structure, while the LW regions displayed a smoother and rather closed surface. Due to these interesting surface features, it is possible to control the deposition of modifiers such as zinc oxide (ZnO) rods growth on wood scaffold in a selective deposition manner. Indeed, after the treatment, we observed a clear difference in the distribution of ZnO mirorods, leading to a wettability difference: EW turned hydrophobic (CA ~ 130°) while LW remained hydrophilic (CA ~ 60°). This facile lithography-free technique opens up new opportunities to functionalize wood surfaces, and develop lignocellulosic material with advanced properties.
 Parker, Andrew R., and Chris R. Lawrence. Nature 414.6859 (2001): 33.