Surfactant-Mediated Collapse of Liquid Marbles and Directed Assembly of Particles at the Liquid Surface

Abstract

Extensive research is being devoted to both the fundamental and applied aspects of liquid marbles (LMs). However, influence of the surface tension of the liquid substrate on the stability of the LMs and LM-mediated capillary interaction remains unexplored. In this work, we unveil the role of the surface tension of the liquid substrate on the collapse of multilayered LMs and apply this knowledge for realizing a dense planar assembly of microparticles triggered by LM-mediated capillary interactions. Experiments and analysis show that the required surface tension for the collapse is dependent on the volume of the LMs. The larger LMs are less stable, and thus collapse at a higher surface tension than that required for smaller LMs. The results are explained on the basis of the balance between surface tension forces acting on the LM (F s) and its weight (F w). Force analysis reveals that the collapse of the LM on the liquid substrate occurs when the surface tension force approaches to its weight, that is, when F s ≈ F w. This has been verified for LMs having volume in the range 6–10 μL. The experiments with different surfactants (an anionic and a cationic) lead to similar results which indicate that the collapse condition of the LMs is mainly dependent on their weight and the surface tension of the liquid substrate. Further, we demonstrate the LM-mediated assembly of particles at the liquid surface, and interestingly, the LM can be collapsed once the assembly is completed, leading to a denser well-packed assembled structure. We believe that the presented results could provide new insights in the fields of microfluidics, particle patterning, and assembly.

Effect of Core Liquid Surface Tension on the Liquid Marble Shell

“…Stability of a liquid marble decreases with the surface tension. [51,67,68] In our experiment, a water liquid marble was highly stable on a clean glass substrate, whereas a liquid marble at a surface tension of 43 mN m −1 had lower stability. Liquid marbles at a surface tension of 43 mN m −1 collapsed within 70-120 s after transferring to the glass substrate (see Supporting Information).…”

Responsive Photonic Liquid Marbles

“…42 mN m −1 for a broad concentration range) than pure water. Second, their small size (20–30 nm for individual NPs, few hundred nm for aggregates) ensures a reduction of light scattering and thus optically clear LMs, contrary to other particles typically utilized (Supporting Information, Figure S2). We create drops (volume 2 μL) of an aqueous HPC solution at c init =55 wt % (Supporting Information) that is in the biphasic regime, where an isotropic and a cholesteric phase co‐exist (Supporting Information, Figure S3), in accordance with earlier work …”

Long‐Lived Liquid Marbles for Green Applications

“…Natural [8,28,30,[61][62][63][64] Fluorinated [1,47] Salinized [65][66][67] 17-50 138°-144°1 60°-165°-Silica Salinized [1,5,43,44,47,[67][68][69][70][71] Fluorinated [57,64,[71][72][73] 0.01-0.5 113°1 20°-156°P olytetrafluoroethylene (PTFE) Untreated [11,18,22,27,55,62,68,69,[74][75][76][77][78][79] 0.13-100 100°-112°P olyvinylidene fluoride (PVDF) Untreated [7,8,61,62,74,[80][81][82] 0.13-0.36 100°P olyethylene (PE) Untreated [31,…”

“…[142] The presence of a multilayer shell also prevents critical wetting from occurring, allowing for LM transfer onto a liquid surface. [82,115,123] This is especially useful in environments where the likelihood of wetting is high due to the presence of surfactants. A multilayer LM can allow wetting of the outer layers without compromising its structural integrity, exhibiting a greater resistance to collapse than a monolayer LM which cannot allow the wetting of its only particle layer.…”

“…A multilayer LM can allow wetting of the outer layers without compromising its structural integrity, exhibiting a greater resistance to collapse than a monolayer LM which cannot allow the wetting of its only particle layer. [82] Nevertheless, it is advantageous in terms of LM surface property controllability to utilize a monolayer, ensuring a uniform shell thickness, which cannot be guaranteed with multilayered LMs. [6,24,138] It is not always possible to achieve a tightly packed uniform structure with multilayered LMs, which is possible with a monolayer, where a critical jamming state occurs.…”