Singha Pradip, Swaminathan Sreena, Yadav Ajeet Singh, Varanakkottu Subramanyan Namboodiri
Langmuir. 2019 Apr 2;35(13):4566-4576. doi: 10.1021/acs.langmuir.8b03821. Epub 2019 Mar 14.
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) and its weight ( F). 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 ≈ F. 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.
大量研究致力于液滴弹（LMs）的基础和应用方面。然而，液体基质的表面张力对液滴弹稳定性和液滴弹介导的毛细管相互作用的影响仍未得到探索。在这项工作中，我们揭示了液体基质表面张力在多层液滴弹坍塌中的作用，并将这一知识应用于实现由液滴弹介导的毛细管相互作用触发的微粒致密平面组装。实验和分析表明，坍塌所需的表面张力取决于液滴弹的体积。较大的液滴弹稳定性较差，因此在比小液滴弹所需更高的表面张力下坍塌。基于作用在液滴弹上的表面张力（F）与其重量（F）之间的平衡对结果进行了解释。力分析表明，当表面张力接近其重量时，即当F≈F时，液滴弹在液体基质上发生坍塌。对于体积在6 - 10μL范围内的液滴弹，这一点已得到验证。使用不同表面活性剂（一种阴离子型和一种阳离子型）进行的实验得出了类似的结果，这表明液滴弹的坍塌条件主要取决于其重量和液体基质的表面张力。此外，我们展示了液滴弹在液体表面介导的颗粒组装，有趣的是，一旦组装完成，液滴弹就可以坍塌，从而形成更致密的紧密堆积组装结构。我们相信，所呈现的结果可以为微流体、颗粒图案化和组装领域提供新的见解。
