Biomimetic Fabrication Of Fiber Based Hairy Surfaces With Underwater Air-retaining Properties

The ability of some plants and insects to maintain the air layer underwater is another emerging research area after the lotus effect.This kind of ability often associated with hydrophobic hairy surfaces.Specially,plant Salvinia molesta was reported by Barthlott et al.in year 2010 for its high air layer persistence that can lasted for more than ten days or even a month.The salvinia leaf surface is dominated with elastic egg-beater shape hairs coated with nanostructure of wax crystalloids like the lotus leaf which provide hydrophobicity.Except for the terminal cells of each hair without any wax which form hydrophilic patches.This combined hydrophilic-hydrophobic elastic hairy surface endow good air retaining ability under water.The biomimicking research of salvinia is in the ascendant.In this research,fiber based materials were used.According to the structure and properties of the Salvinia molesta leaf surface,hairy surfaces were fabricated by electrostatic flocking method and terry fabrics were selected,and both were hydrophobic modified to mimic the underwater air retaining ability.The potential applications of the bionic samples were investigated.On the base of references,the surface structure,hydrophobicity and the adhesion of hydrophilic tips of the salvinia leaf were analyzed.Some polymer material samples(nylon velcro)and some pile fabrics which have resemblance in structure with salvinia surface were modified for hydrophobicity,and their surface hydrophobic performance and underwater air persistence were analyzed.It was found that in spite of the the resemblance in structure and the similarity in hydrophobicity,the polymer material samples did not have good air retaining ability.The pile fabric samples showed great hydrophobicity,however,when the pile height exceeded certain level,they tilted under water which would compromise air persistence.Through the analyze of the preliminary experimental results,it was speculated that a surface with thin and dense hairs in addition of proper height and support strength should be in favour of air retaining.Thus,the electrostatic flocking technique was used to fabricate samples mimicking the underwater air retaining ability of salvina leaves.This method was easy and tunable forsurface flock size and density.In the meantime,the air volume change test was designed,four values: the volume of the total air layer Vtotal,the air volume left on the sample after a certain period of time Vleft,the volume of air loss as bubbles Vbubble and the volume of air that dissolved into water Vdissolved were recorded & calculated to analyze the change of the air layer under water more clearly.Flocking samples were fabricated with flock fibers differed in material and size,and the flock density was adjusted by flocking time.Then the flocking samples were hydrophobically modified.Through the comparison between different flocking samples,it can be concluded that the air-retaining time extended with the increase of flock height as a higher flock height would increase Vtotal and thus extend the air retention.A relatively small flock diameter and high density of flocks on a substrate which decrease the gap between flocks and maximize the capillary pressure against water penetration would benefit the air retention.However,the flocks were commonly designed with synchronous diameters and heights,when the flock size increased to a certain level where the effect of the enlarged gap surpassed the effect of the height increasing,water penetrated between flocks much more easily,leading to bad air retention.The best performance flocking sample 5C(nylon flock,flock diameter 0.022 mm,flock height 0.9mm,flock density 500-700/mm2)showed comparable air retaining ability to salvinia leaves.When the test was ended at 600 h,the salvinia-flat sample and flocking sample 5C had 0.4 m L/25cm2(18% of Vtotal)and 0.5 m L/25 cm2(25% of Vtotal)air left,respectively.At last,the water resistance on flocking samples with air layer and without air layer was measured through viscosimeter method.It was found that the air layer on flocking samples have drag reduction effect.When the flock size increased to a certain level,in addition to the decreasing of the flock density,there were more tilting of flock fibers.Thus,flocking samples with step structure was designed by planting flocks of different length on same substrate,in hope of the shorter flock would enhance the air retaining ability of the higher flock.It was found that the double-layer step flocking can enhance the stability of air-water interface by slowing down the air bubble loss.The mechanism of this phenomenon was preliminarily analyzed.Compared with double-layer step structure,triple-layer step structure did not show better performance except slight enhance of stability at the early stage of immersion.To prepare hydrophilic tips like those on Salvinia molesta,a dipping method was designed and a triple-layer step flocking sample with hydrophilic tips(density 350-500/cm2,area ratio 4-7%)comparable with salvinia in density and area ratio(density 233 ± 29/cm2,area ratio 2.2% ±0.9%)was fabricated.The stability of air-water interface was enhanced with air bubble loss of the sample decreased about 29% compared with the none hydrophilic tip sample with the same structure.It was proved that the hydrophilic tips can enhance the stability of air-water interfac.On the other hand,considering the eggbeater shape of the hair of Salvinia molesta,may provide extra deformation potential energy against water penetration,terry fabric(looped fabric)was studied for biomimicking.Different loop structure was selected and analyzed for air retention.It was found that the loop yarns composed by fine fibers may enlarge surface roughness and air volume retained which was important for under-water air retention.Higher loop would benefit the total air volume retained.A loop structure standing vertically rather than leaning aside would increase the defense against water penetration.A straight line up loop arrangement should be avoided as the groove space between loop lines would enhance air loss.The best performance samples F1(cotton,arch loop profile,loop height 1.25-1.31 mm,loop yarn diameter 0.17-0.22 mm,distance between loop 0.45-0.89 mm,fabric thickness 1.74-1.80 mm)and F3(viscose,whole circle loop profile,loop height 1.67-1.74 mm,loop yarn diameter 0.20-0.25 mm,distance between loop 0.46-0.56 mm,fabric thickness 2.05-2.13 mm)which showed comparable hydrophobicity and air retention ability with salvina leaves can provide maximum air of 1.3 L/m2 and 1.5 L/m2 to generate 13 N/m2 and 15 N/m2 buoyancy force,respectively.After 500 h immersion,F1 and F3 can still have 70% and 66% air left,respectively.It was considered that in addition to the air on the surface,there were air trapped in between the fibers of the loop and the substrate and inside the loop.Through the underwater thermal insulation test designed,it was found that F1 and F3 with air-retaining ability showed much lower heat loss rate than samples without hydrophobic modification.Such fabrics or such structure fabrication could be potentially utilized in lifesaving appliance.Finally,inspired by the fast and selective oil sorption of salvinia leaf,electrostatic flocking samples which was previously designed to mimic the air retaining ability of Salvinia molesta leaf was further studied for oil sorption and the potential application in oil spill clean up.It was found that with proper hydrophobic modification the flocking sample selectively absorbed oil from oil-water mixture without absorbing any water,and showed rapid absorption and large sorption capacity,the sample maintained floating on water which would be easy for retrieve.With certain padding pressure applied,>70% of the oil could be squeezed out and the sample could be reused for several times.The finding provided an innovative approach for potential sustainable application of flocking fabrics or other similar hairy surface fabrics.