Jimidar Ignaas S M, Sotthewes Kai, Gardeniers Han, Desmet Gert, van der Meer Devaraj
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium.
Mesoscale Chemical Systems group, MESA+ Institute and Faculty of Science and Technology, University of Twente, P. O. Box 217, 7500AE Enschede, The Netherlands.
Soft Matter. 2022 May 18;18(19):3660-3677. doi: 10.1039/d2sm00432a.
The vibration dynamics of relatively large granular grains is extensively treated in the literature, but comparable studies on the self-assembly of smaller agitated beads are lacking. In this work, we investigate how the particle properties and the properties of the underlying substrate surface affect the dynamics and self-organization of horizontally agitated monodisperse microspheres with diameters between 3 and 10 μm. Upon agitation, the agglomerated hydrophilic silica particles locally leave traces of particle monolayers as they move across the flat uncoated and fluorocarbon-coated silicon substrates. However, on the micromachined silicon tray with relatively large surface roughness, the agitated silica agglomerates form segregated bands reminiscent of earlier studies on granular suspensions or Faraday heaps. On the other hand, the less agglomerated hydrophobic polystyrene particles form densely occupied monolayer arrangements regardless of the underlying substrate. We explain the observations by considering the relevant adhesion and friction forces between particles and underlying substrates as well as those among the particles themselves. Interestingly, for both types of microspheres, large areas of the fluorocarbon-coated substrates are covered with densely occupied particle monolayers. By qualitatively examining the morphology of the self-organized particle monolayers using the Voronoi approach, it is understood that these monolayers are highly disordered, , multiple symmetries coexist in the self-organized monolayers. However, more structured symmetries are identified in the monolayers of the agitated polystyrene microspheres on all the substrates, albeit not all precisely positioned on a hexagonal lattice. On the other hand, both the silica and polystyrene monolayers on the bare silicon substrates transition into less disordered structures as time progresses. Using Kelvin probe force microscopy measurements, we show that due to the tribocharging phenomenon, the formation of particle monolayers is promoted on the fluorocarbon surface, , a local electrostatic attraction exists between the particle and the substrate.
相对较大颗粒的振动动力学在文献中已有广泛论述，但对于较小搅拌珠自组装的类似研究却较为缺乏。在这项工作中，我们研究了颗粒特性以及底层基底表面特性如何影响直径在3至10μm之间的水平搅拌单分散微球的动力学和自组织。搅拌时，团聚的亲水性二氧化硅颗粒在平坦的未涂层和氟碳涂层硅基底上移动时会局部留下颗粒单层的痕迹。然而，在具有相对较大表面粗糙度的微加工硅托盘上，搅拌的二氧化硅团聚体形成了分离带，这让人想起早期关于颗粒悬浮液或法拉第堆的研究。另一方面，团聚较少的疏水性聚苯乙烯颗粒无论底层基底如何都会形成密集占据的单层排列。我们通过考虑颗粒与底层基底之间以及颗粒自身之间的相关粘附力和摩擦力来解释这些观察结果。有趣的是，对于这两种类型的微球，氟碳涂层基底的大片区域都被密集占据的颗粒单层覆盖。通过使用Voronoi方法定性检查自组织颗粒单层的形态，可以了解到这些单层高度无序，自组织单层中共存多种对称性。然而，在所有基底上搅拌的聚苯乙烯微球的单层中都发现了更具结构性的对称性，尽管并非所有都精确地排列在六边形晶格上。另一方面，随着时间的推移，裸硅基底上的二氧化硅和聚苯乙烯单层都转变为无序程度较低的结构。通过开尔文探针力显微镜测量，我们表明由于摩擦起电现象，氟碳表面上促进了颗粒单层的形成，即颗粒与基底之间存在局部静电吸引力。
