Abstract:

Objective Because of their wide range of applications, surfaces with switchable wettability between superhydrophilicity and superhydrophobicity brought about by external stimuli have attracted intensive research attention. However, almost all of these surfaces are responsive to only single external stimuli, which limits the applications of wettability switching surfaces. Compared with single stimuli surfaces, the surfaces with dual or multiple stimuli have better environmental adaptability. Therefore, superhydrophobic surfaces with dual or multiple stimuli responsiveness have become a research focus. This study is proposed to prepare superhydrophobic cotton fabrics with ultraviolet/ammonia dual responsiveness.

Method Ferrous sulfate and ethanedioic acid were used as the raw materials. FeC₂O₄ was prepared and then was calcined in a muffle furnace at 300 °C for 3 h before preparing ferric oxide particles. The ferric oxide particles, anhydrous ethanol, and stearic acid were added into a flask and stirred at ambient temperature for 0.5 h, and then mixed with the anhydrous ethanol suspension of chitosan before hydrophobic suspension was obtained. Cotton fabrics were dipped in the hydrophobic suspension and dispersed in an ultrasonic bath for 10 min, followed by drying in an oven at 60 °C to obtain the superhydrophobic cotton fabrics. Their morphologies and surface chemical compositions were analyzed by Fourier transform infrared spectroscopy (FT-IR), scanning electron micro-scopy (SEM) and energy dispersive spectroscopy (EDS). The influences of ultraviolet and ammonia on the wettability of superhydrophobic cotton fabrics were investigated, and the influence of temperature on the recovery of their superhydrophobicity was studied.

Results The X-ray diffraction (XRD) analysis revealed that the prepared ferric oxide was γ-Fe₂O₃ (Fig. 1). Water contact angle measure results showed that the prepared cotton fabric possessed good superhydrophobicity, and its water contact angle was 153.94° (Fig. 5). SEM analysis showed that γ-Fe₂O₃ particles and chitosan formed nanoscale and microscale rough structure on cotton fibers (Fig. 3). FT-IR and EDS analysis revealed that chitosan and stearic acid with low surface energy covered on the surface of cotton fibres (Fig. 2 and Fig. 3(b)). The superhydrophobicity of the cotton fabrics was obtained by combining micro-nano hierarchical rough structure and low surface energy material. After 28 h of ultraviolet irradiation, the as-prepared fabric changed from superhydrophobic to superhydrophilic (Fig. 6), and under the synergical effect of ultraviolet irradiation and H₂O₂ solution, the superhydrophobic cotton fabric converted to superhydrophilic within 7 h (Fig. 7). The above superhydrophilic fabric recovered to superhydrophobicity after standing in the dark for 28 d (Fig. 8). The superhydrophobicity recovery time decreased with the increasing of recovery temperature. In particular, the superhydrophilic surface converted to superhydrophobic when exposed to 120 ℃ for 40 min (Fig. 9). Meanwhile, the superhydrophobic cotton fabric changed from superhydrophobic to superhydrophilic when it was induced by ammonia for 5 s (Fig. 10). The above superhydrophilic fabric also recovered to superhydrophobic at ambient temperature (Fig. 11). The superhydrophobicity recovery time also decreased with increasing recovery temperature. For instance, the superhydrophilic fabric recovered to superhydrophobic when exposed to 80 ℃ for 50 min (Fig. 11).

Conclusion The prepared cotton fabrics possess good superhydrophobicity. Under ultraviolet irradiation and in ammonia atmosphere, the cotton fabrics could change from superhydrophobic to superhydrophilic, and the process is reversable. The superhydrophobicity recovery time is decreased with the increasing of recovery temperature. The proposed preparation method is simple and easy, and it can be easily extended to other surfaces. The fact that superhydrophobic surfaces have the capability to switch the wettability by ultraviolet or ammonia, and has potential applications in oil-water separation, microfluidic switching, drug delivery, and other similar applications.

Key words: superhydrophobic cotton fabric, ferric oxide, ultraviolet, ammonia, responsiveness, wettability conversion