Liquid marbles prepared from pH-responsive self-assembled micelles

Abstract

In this study, we report the assembly of amphiphilic polymeric micelles at the liquid/air interface to prepare liquid marbles for the first time. The polymeric micelles were synthesized from the self-assembly of a fluoropolymer, poly(styrene-co-acrylic acid-co-2,2,3,4,4,4-hexafluorobutyl methacrylate), in a selective solvent. The particle size, morphology and chemical composition of the micelles were determined by dynamic light scattering (DLS), transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. DLS and aqueous electrophoresis revealed the pH-responsiveness of the micelles in aqueous dispersion. Liquid marbles with water volumes varying from 10 μL to 1 mL were formed by rolling water droplets on the micelle powder bed. The increase in water volume led to the shape transition of the liquid marbles from quasi-spherical to a puddle-like shape because of gravity. Fluorescence microscopy was used to observe the morphology of the formed liquid marbles, which confirmed that the micelles were adsorbed at the interface of water and air. The effective surface tension of the liquid marbles decreased with the increasing concentration of NaOH, which was added to the interior water phase. This agreed with the results of droplet roller experiments: the mechanical integrity of the liquid marbles prepared from alkaline solution (pH 10) was relatively poorer than those prepared from acidic solution (pH 2). Moreover, these liquid marbles coated with micelles showed pH-responsiveness when transferred onto the surfaces of aqueous solutions with different pH values. The liquid marbles were relatively stable on the acidic solution, whereas they burst immediately on the alkaline solution with a pH of 10. In addition, apart from water, Gellan gum solution and glycerol could be also successfully encapsulated by the fluorinated micelles to form stable liquid marbles.

The Potential of Liquid Marbles for Biomedical Applications: A Critical Review

“…In literature several works studying and exploring the ability of LM to float can be found – Figure A . Taking advantage from this LM property, applications were developed on sensors field for pH‐sensing, to reveal water pollution, and also on development of floating self‐propelling devices . Generally, pH‐sensing LM were coated with pH‐responsive particles that change their wettability from hydrophobic to hydrophilic with pH variations.…”

“…These LM usually exhibit long‐term stability floating in solutions with a pH above or below a defined value. With the addition of an acid or a base to the supporting liquid, the LM immediately disintegrate due to the increase of the coating hydrophilicity. The pH changes can be visually detected by LM destruction and/or by colorant release.…”

“…Widely range of applications were found for LM, such as oil adsorption and separation, pH and gas sensing, chemical reactions, synthesizing microparticles, revealing water pollution on the water/vapor interface and manipulation of small amounts of liquids (micro‐reactors, micro‐pumps) . The LM use in cosmetics has been also suggested, due to the non‐oily feel imparted to skin by marbles .…”

Liquid Shaping Based on Liquid Pancakes

“…[16] Initially, liquid marbles were so called partly due to their quasispherical shape, which becomes oblate when the volume is increased and thus are sometimes called a "liquid puddle." The general mechanism for successfully deforming a coated drop, either in air or liquid, is to subject the interfacial particles to jamming, [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] in which case the particle shell behaves as a rigid layer endowing the drop with a stable irregular shape. The general mechanism for successfully deforming a coated drop, either in air or liquid, is to subject the interfacial particles to jamming, [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] in which case the particle shell behaves as a rigid layer endowing the drop with a stable irregular shape.…”

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Technically, this is achieved by increasing the particle density through changing the drop's surface area and, in the case of a liquid marble, the approach mainly includes droplet/marble impingement on a powder, [23,31] marble coalescence, [28,32] evaporation from the marble, [25][26][27] and artificial liquid extraction. [33] Given that marble shapes can be well controlled, such structured marbles could serve as smart containers allowing complex manipulations of the internal substances and, hence, a wider variety of results can be expected, compared with applying only spherical or puddle-shaped liquid marbles.

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