A Nano-modified Superhydrophobic Membrane

Graduate Studies Program in Mechanical Engineering, Deparment of Mechanical Engineering, Universidade Federal de Minas Gerais – UFMG, Av. Antonio Carlos, 6627, CEP 31270-901, Belo Horizonte, MG, Brazil Chemistry Program, Centro Federal de Educação Tecnológica de Minas Gerais – CEFET-MG, Av. Amazonas, 5352, CEP 30421-169, Belo Horizonte, MG, Brazil Aviation and Missile Research Development and Engineering Center, AMSRD-AMR-WD-GA, Building 5420, Redstone Arsenal, 35898, USA


Introduction
As discussed by Du et al. 1 , knowledge of interfacial water structures near hydrophobic surfaces is essential for understanding important surface problems involving water.The interaction between molecules of water and the surface can lead to self-cleaning surfaces.The idea of self-cleaning surfaces is not new.Nature has being producing these surfaces for centuries; the leaves of the lotus plant (Nelumbo nucifera) and wings of butterflies are some examples of natural self-cleaning surfaces.According to Lum et al. 2 , the interest in self-cleaning surfaces is being motivated by possible industrial applications, e.g.solar energy panels including photovoltaics, exterior architectural glass, anti-freeze surfaces, and medical devices ranging from blood vessel replacement to wound management.
As argued by Zhai et al. 3 , a superhydrophobic surface is able to repel water droplets completely; such surfaces exhibit water droplet advancing contact angles (CA) of 150° or higher.Likewise Lafuma and Quéré 4 pointed out that hydrophobicity and superhydrophobicity are related to the surface roughness since it brings as a consequence the increase on surface area, which leads to more loci for air-trapping.The two most relevant models that attempt to explain hydrophobicity are Cassie's and Wenzel's models.Cassie's model for hydrophobicity is based on the idea of water drops being settled over air bubbles, while the Wenzel's model associates hydrophobicity to surface roughness.However, none of them is capable of explaining hydrophobicity in its completeness.Miwa et al. 5 went further as they argued that surface roughness should be associated to surface energy in order to create a superhydrophobic behavior.
Several different approaches can be applied to artificially create hydrophobic and superhydrophobic surfaces.One option is to create micro texturized surfaces.This idea was tested by Lafuma and Quéré 4 with relative success.Although they were able to measure water CA of 164°, they realized that microtextures can be filled with water, especially when condensation is present.A similar approach was suggested by Lepore et al. 6 , as they proposed surface modification of polystyrene plates by electrical discharge This paper focuses on the development of hyper-hydrophobic micro/nano-membranes produced by electrospinning of PS/DMF solutions doped with nanoparticles.These membranes can be applied to glasses, windshields, or walls of surgery centers.Another important application of these membranes is on natural fibers and natural/green composites.Moisture absorption is a relevant issue for natural composites.Hyper-hydrophobic membranes can be applied to these natural composites as an "extra" protective layer to reset the barriers against moisture.

Material and Experimental Procedures
PS (Mw: 190,000 g.mol -1 ), DMF (HPCL grade), ZnS and CdS were purchased from Sigma-Aldrich, nanosilica was supplied by Nissan Chemical, while the graphene was obtained using the procedure described in Ávila et al. 10 .The PS/DMF solutions were prepared under mild stirring (200 RPM) at a temperature of 40 °C.Once the PS solution reached the room temperature, particles were dispersed into the solution by ultra-sonication (at 20 kHz) for 30 minutes.The electrospinning technique described by Ko and Gandhi 11 and Ramakrishna et al. 12 was employed in this research.The electrospinning device is composed by different components, i.e. a high voltage supplier (Gamma High Voltage Research RR-30-150W, USA), a syringe pump (Harvard Apparatus, USA), a capillary tube with a stainless steel need (G18 blunted, diameter of 1.2 mm), and a collecting drum with a diameter of 100 mm.
To m e a s u r e t h e w a t e r c o n t a c t a n g l e , a n adjustable micropipette (0.1 µL-1.0 µL) was employed as the source of distillated water.The fibers/membranes morphology was investigated using a Quanta 200 -FEG -FEI -2006 scanning electronic microscope.The public domain software for image-processing 13 was used for water contact angle measurements.The solution viscosity was measured using Cole-Parmer EW-98936 viscometer.Since there is no consensus on which polymer concentration can lead to the best results in terms of water CA (Jiang et al. 8

Results and Discussion
As commented by Kang et al. 9 , PS is chemically hydrophobic.Carré 14 attributed this hydrophobicity to the PS low surface energy.His statement was based on Wenzel's model.However, the water CA is not enough to define PS as a superhydrophobic material.To enhance the "natural hydrophobicity" of PS surfaces, these surfaces have to be modified to increase their roughness.These changes can be made by dispersing nanoparticles into the electrospinning solution.Table 1 summarizes each set of experiments performed and their main parameters with respect to the electrospinning process.The electrical field density applied was kept constant and equal to 150 KV/m, i.e. 15 KV and distance needle tip to collector of 0.1 mm (100 mm), to all nine sets of experiments.

Group ID PS [wt. (%)] Nanosilica [wt. (%)] Graphene [wt. (%)] CdS [wt. (%)] ZnS [wt. (%)] Flow rate [µL/min]
A summary of all water CA is shown in Figure 2. The statistical analysis (Levene's test) indicates that the nine sets are significantly different for a critical p-value of 0.05.The large variations on water CA measurements can be attributed to the random distribution of fibers that creates a very rough surface.As the hydrophobicity increases, the interface between the beginning of the water droplet and the end of the surfaces becomes more defined and the standard deviation on water CA measurements decreases.As it can be noticed, group #1 presented a higher water CA (147.96° ± 8.51°) than group #2 (143.01°± 7.88°).This can be explained by the increase on viscosity, as shown in Figure 1.According to Ko and Gandhi 11 , less viscous solutions lead to smaller fiber diameters.Furthermore, as discussed by Carré 14 , smaller fiber diameters increase the surface roughness and consequently the water contact angle.
The addition of nanoparticles created the conditions for larger water CA in all cases, when compared against the two sets of experiments (groups #1 and #2) without nanoparticles.In group #3, the addition of graphene led to an increase on water CA (152.09° ± 5.35°).A high resolution scanning electron microscopy (HRSEM) analysis (Figure 3a) reveals a rough surface with discontinuous groove marks of 120 ± 5 nm in length and 7 ± 0.8 nm in width.The roughness associated to the PS low surface tension could explain the increase on water CA.Although the results can suggest that an increase on graphene concentration can generate even better values of water CA, there is a drawback for using graphene.As the graphene density is too low (<0.5 g.cm -3 in the "powder" form), if we increase the graphene weight percentage, we likely produce a much higher viscous solution.For practical purposes, the 0.5 wt.(%) seems to be the graphene upper bound for 20/80 PS/DMF solutions.
The water CA values for Groups #4 (149.79°± 7.85°) and #5 (153.09°± 6.58°) are on superhydrophobic range.Nonetheless, there is no linear relationship between the nanosilica content and the water CA.This non-linearity can be explained by differences in the fibers morphology.By observing Figures 3b, c, it is possible to notice a large increase in porosity for the 1.0 wt.(%) nanosilica.The increase in porosity leads to changes in fiber diameters and shapes.Unfortunately, these changes are uncontrollable and therefore unacceptable for industrial applications.
Groups #6 through #9 reveals a nonlinear increase on water CA.The addition of cadmium nanoparticles led to a water CA of 151.22° ± 6.80°, while the Zn addition produced conditions to water CA of 153.20° ± 7.47°, 163.49° ± 4.93°, and 168.22° ± 3.19° for 5 wt.(%), 10 wt.(%) and 15 wt.(%), respectively.The increases on water CA can be attributed to the presence of nanoparticles on the fibers' surface (See Figures 3d through 3g).The differences on Cd and Zn sizes (284.39 ± 8.71 nm and 323.34 ± 6.98 nm) and their distribution could be the reason for the differences in WCA.Although cadmium fibers could be classified as superhydrophobic, environmental issues are a serious concern.The additional increase on water CA on Groups #8 and #9 could be explained by the agglomeration of Zn Figure 1 shows the dynamic viscosity for each set of experiments.As it can be noticed, as rotation increases (high shear rates), the dynamic viscosity decreases.However, as discussed by Guerrini et al. 15 , during the electrospinning process small shear rates are developed because of the small flow rate.To keep the same electrostatic forces in all cases, the flow rate for group #2 had to be increased since this group had a higher dynamic viscosity.This happens because group 2 has a 35 wt.(%) of PS.The same flow rate was used in group #1 for comparison purposes.In groups #3 through #9, the small variation on dynamic viscosity can be attributed to the presence of nanoparticles dispersion.the PS surface.Contrary to results previous published in literature, the PS/DMF weight ratio of 20/80 lead to water contact angles (WCA) of 148°, which is higher than the one obtained for the 35/65 ratio, i.e. 143°.The WCA for membranes with 0.5 wt.(%) of graphene reached 152°, 149°-153° with the addition of nanosilica, 151° with 5.0 wt.(%) CdS, and 153°, 163° and 168° with the addition of 5 wt.(%), 10 wt.(%) and 15 wt.(%) of ZnS, respectively.The fiber morphology was also affected by the addition of nanoparticles.Continuous axial groove marks were formed with the addition of graphene, while the addition of nanosilica around 1.0 wt.(%) generated non-homogeneous fibers regardless of the cross section and porosity.The addition of Cd and Zn nanoparticles generated "nano-clusters" on fibers' surface, which altered the overall roughness and consequently the WCA.nanoparticles on the fibers' walls.These agglomerations are on the order of 378.92 ± 8.78 nm and 561.31 ± 11.94 nm, for the 10 wt.(%) and 15 wt.(%), respectively.With the increase of nanoparticles "clusters", roughness is enhanced which leads to a higher water contact angle.Finally, given the water CA values obtained, it is possible to categorize Group #9 (addition of 15 wt.(%) of ZnS) as a hyper-hydrophobic membrane.

Conclusions
PS fibers doped with nanoparticles were prepared by electrospinning.In contrast to results previous published in literature, the PS/DMF weight ratio of 35/65 did not lead to higher water contact angles.This fact can be attributed to the presence of non-evaporated solvent into suggested 25 wt.(%), Kang et al. 9 employed 35 wt.(%)), this research selected the two extreme limits: 35 wt.(%) as upper bound and 20 wt.(%) as lower bound.