and loaded the boat with heavy rubber (density 1.7gcm−3). The volume of the boat was 8.0 cm3 . According to Archimedes’ principle, the loading capacity of this boat on water surface should be 8.0 g. However, we found that the maximum loading capacity for this boat was 11.6 or 12.2 g if the boat weight is included. This suggests that besides the buoyancy force associated with the boat volume of 8.0 cm3 , there is an extra buoyancy force to support the extra load- ing of 4.2 g. Fig. 8 shows the floating behavior of a superhydrophobic fabric boat with a maximum loading. As can be seen, the boat still floated even when its upper edges were below the water surface, which can be ascribed to the extremely low surface energy of the fabric. Therefore, one contribution to the extra buoyancy force is the surface tension force related to the upper perimeter of the boat. Another contribution to the extra buoyancy force comes from the plastron effect [38,39]. The trapped air film surrounding the fabric outer surface provides additional displaced volume of water, and thus extra buoyancy force.
For practical applications, the durability of the superhydrophobic surface is important. In our study, the affinity between the (PAH/SiO2 )5 multilayers and the cotton fiber was tested by immersing the fabric into a sonication bath containing ethanol, which can completely wet the superhydrophobic fabric. After continuous sonication for 1 h, no significant changes in the contact angles were found. In addition, the wash durability test revealed that the contact angle hysteresis of the cotton fabric assembled with (PAH/SiO2)5 multilayers still remained lower than 10◦ after 10 cycles of home laundering, and the static contact angle remained above 150◦ even after 30 cycles (Fig. 9).