Remotely Controllable Miniature Reactors: Magnetic Liquid Marbles

Multifunctional materials have gained a lot of interest in recent years, especially in the fields of biomedical engineering, catalyst, self-cleaning surfaces, gas sensing or drug delivery systems [1]. Liquid marbles, which are formed by rolling the water droplets over hydrophobic nano or micro particles, provide a unique manipulation method of water droplet with its non-wetting and non-stick properties. At the same time, stimuli-responsive liquid marbles response with the change of temperature, pH, magnetic field, UV radiation and available to use in various applications such as micro reactors or micro fluidics [2]. Numerous liquid marbles prepared by using magnetic particles, were studied in previous works. UV triggered magnetic nanoparticles have been synthesized providing a remotely triggered rupture of a liquid marble [3]. Fe₂O₃nanoparticles have also been dispersed in the core ionic liquids of PTFE particle based liquid marbles to provide magnetic manipulation [4].

Here we studied a new perfluorinated ligand to surface modification of magnetite nanoparticles and formed liquid marbles with water as core liquid. First, approximately 6 nm sized iron oxide nanoparticles were synthesized via co-precipitation of FeCl₃ and FeSO₄salts in water by adding NaOH solution. As-synthesized nanoparticles showed superparamagnetic properties and have 60 emu/g of saturation magnetization value. Then, water-soluble magnetic particles were washed 3 times with ultrapure water and redispersed in methanol. Perfluorinated ligand was added to the solution and interacted with particles under 5 minutes of sonication. To obtain liquid marbles, particles were dried, ground and various volumes of water droplets were rolled over this hydrophobic powder. In order to analyze the morphology, size and size distribution of the magnetite nanoparticles, High Resolution Transmission Electron Microscopy (HR-TEM) analysis was performed. Also, Dynamic Light Scattering (DLS), Vibrating Sample Magnetometer (VSM) and X-ray diffraction (XRD) were used to implement a detailed characterization of nanoparticles. Contact angle measurements were performed by the sessile drop method (Krüss DSA 100). Moreover, stabilization of liquid marbles with various volumes of core liquids were monitored by visualizing the marbles with a CCD (charge coupled device) camera for a certain time period.

Liquid marbles were able to keep their shape up to 75 minutes. The liquid marble that obtained with 5 microliters of water showed >150° of contact angle value on a glass slide. A linear relation was determined between the height and diameter values, that means formation of approximately spherical shaped small magnetic liquid marbles and gravitationally flattened puddles were obtained when the large volume of water droplets was used to form liquid marbles. Magnetic manipulation of marbles was performed by a permanent magnet and it was determined that the velocity of marbles under a magnetic field were considerably high. When a marble was on a glass surface and a magnet was approached towards under of the marble, magnetite nanoparticles got closer to the glass surface and top of the marble was opened. This opening/closing process of liquid marbles was reversible and can be repeated for more than 10 times. As a reactor, opening and closing process allows adding various reactants after the formation and transportation of marble. In this work, a gelation reaction was carried out as a model reaction. This gelation reaction provided a mechanical robustness for marbles. Thus, stability of marble was increased and it was possible to pick it up and press or stick a needle through it without rupture of the marble.

REFERENCES

[1] G. McHale, M. I. Newton, Soft Matter, 2011, 7, 5473.

[2] J. Sun, W. Wei, D. Zhao, Q. Hu, X. Liu, Soft Matter, 2015, 11, 1954.

[3] L. Zhang, D. Cha, P. Wang, Adv. Mater., 2012, 24, 4756–4760.

[4] S. Zhang, Y. Zhang, Y. Wang, S. Liu, Y. Deng, Phys. Chem. Chem. Phys., 2012, 14, 5132–5138.