Multi‐Responsive Janus Liquid Marbles: The Effect of Temperature and Acidic/Basic Vapors

1 wileyonlinelibrary.com www.particle-journal.com www.MaterialsViews.com C O M M U N IC A IO N wax and hydrophobic Fe 3 O 4 nanoparticles (wax/Fe 3 O 4 powder), while the white powder was a mixture of decanoic acid (DA) and heptylamine (HA)-modifi ed titanium oxide nanoparticles (DA–TiO 2 /HA–TiO 2 , see Figure S1, Supporting Information). To prepare such a Janus liquid marble, a magnetic liquid marble was positioned at the lower surface of a glass plate using a magnet bar. By moving the magnet bar toward the liquid marble, the bottom surface of the magnetic liquid marble was opened (Figure 1 b, left). The liquid marble was held at the open state and nonmagnetic DA–TiO 2 /HA–TiO 2 powder was then brought into contact with the exposed liquid hemisphere. As a result, the DA–TiO 2 and HA–TiO 2 particles attached to the water surface, leading to the formation of a Janus liquid marble (Figure 1 b, right). It is noted that Janus liquid marbles have been prepared by collision and coalesce of two liquid marbles, which are prepared from different powders. [ 7a ] Here, our method is completely different from the previous art, and it is more controllable in droplet size and shell profi le. Stimulusinduced wettability change of the powders will allow the Janus liquid marble to rupture under a stimulus particular to each type of powder. The Janus liquid marble ruptured upon IR irradiation when the temperature-responsive wax/Fe 3 O 4 semishell contacted with the glass substrate (Figure 1 c and movie S1, Supporting Information), and ruptured upon exposure to either ammonium hydroxide or acetic acid vapor when the pH-responsive DA–TiO 2 /HA–TiO 2 semi-shell contacted with the glass substrate (Figure 1 d and movies S2, S3, Supporting Information). Hydrophobic Fe 3 O 4 nanoparticles used in this study were synthesized by coprecipitation of Fe(II) and Fe(III) salts in an ethanol–water solution with ammonia in the presence of a fl uorinated alkyl silane. [ 1c ] The as-obtained Fe 3 O 4 particles had a diameter of ≈10 nm and exhibited a superparamagnetic behavior (see Figure S2, Supporting Information). The temperature-responsive wax/Fe 3 O 4 powder was prepared by mixing wax (melting point, 53–57 °C) with the Fe 3 O 4 particles at 70 °C, which was then grinded into a fi ne powder in liquid nitrogen. Because of the hydrophobicity of both wax and Fe 3 O 4 nanoparticles, the apparent contact angle of a water drop placed on a bed of the wax/Fe 3 O 4 powder was as high as 154 ± 2° (see Figure S3, Supporting Information). Like magnetic liquid marbles reported in our previous work, [ 1c ] liquid marbles prepared with the wax/Fe 3 O 4 powder ( Figure 2 a) had magnetic-fi eld-responsive features. Unlike the previously reported magnetic liquid marbles, the waxcontaining liquid marble ruptured when it was irradiated by IR light due to the melting of the wax in the magnetic shell (Figure 2 b and Movie S4, Supporting Information). IR thermography imaging was employed to measure the temperature of the marble surface. Immediately before the liquid marble Dr. Z. Xu, Dr. Y. Zhao, Prof. T. Lin Institute for Frontier Materials Deakin University Geelong , Victoria 3216 , Australia E-mail: yan.zhao@deakin.edu.au; tong.lin@deakin.edu.au Prof. L. Dai Center of Advanced Science and Engineering for Carbon (Case4Carbon) Department of Macromolecular Science and Engineering Case School of Engineering Case Western Reserve University Cleveland , OH 44106 , USA