2020年9月1日

一种聚合物纳米纤维，比人类头发的百分之一还小，安装在MEMS机械测试设备上。插图显示了两个垂直放置的装置，这样可以在接触的交叉点同时测量附着力和摩擦力。伊利诺伊大学香槟分校(University of Illinois at Urbana-Champaign)的研究人员使用一种小到可以装在大头针头上的设备，获得了关于聚合物纤维纳米级特性的新知识，这些知识可以为由随机细丝网络组成的产品的设计和制造提供信息，比如旨在阻止外来颗粒进入我们肺部的强大过滤器。伊利诺伊大学航空航天工程系博士后学者Debashish Das说:“在生物和生物工程系统中，相互连接的细丝网络无处不在，比如结缔组织、蜘蛛网、组织生长的支架，以及空气过滤器等消费品。”“这项研究为粘附和摩擦在纳米长度尺度上的耦合方式提供了直接的实验见解。相似材料的纳米级纤维相互间粘附性强，分离困难。而且，即使它们被强行分开，它们也会自发地粘在一起。获得对这些现象的实验见解，可以对设计坚固、弹性和坚韧的软纳米纤维网络产生直接影响。”达斯解释说，当我们在微观和纳米尺度上检查纤维和其他表面时，景观发生了变化。“随着我们从肉眼可见的宏观长度尺度越来越小，到微观和纳米长度尺度，颗粒和纤维的表面积与体积相比下降得更慢，所有东西都变得更粘。” In a network of crisscrossing nanofibers with millions of junctions, Das conducted experiments to find out what happens at one of the overlapping junctions and to measure the force required to pull or slide two fibers apart. The diameter of just one of his nanofibers is more than one hundred times smaller than a human hair. "To understand what happens in the network at the macro scale, which is potentially comprised of billions of nanofibers, first we need to understand the mechanical phenomena at the junction where two nanofibers cross," he said. Experimenting with nanoscale fibers requires specialized micro-sized devices. Das designed and fabricated tiny machines—Micro-Electro-Mechanical Systems, or MEMS—that are smaller than one millimeter in size. "In a previous study, we used a MEMS device to stretch a single collagen fiber," he said. "In this study, we coupled two MEMS devices oriented orthogonally to push two fibers together and then separated them by sliding. While doing so we were able to simultaneously measure the force due to adhesion and due to friction. This was the first time such complete measurements were made possible for nanoscale fibers. "From our experimental measurements, we calculated the size of the contact area that is formed between the two nanofiber surfaces at their junction. As we applied a sliding force, the contact started peeling until the sliding force suddenly dropped and an instability occurred, which shows how strong adhesive properties can be at the nanoscale." Das said, "A key finding from our experiments was that the critical sliding force divided by the contact area was equal to the shear yield stress of the polymer . As we pull or stretch a polymer, at a particular stress, it will start deforming plastically and won't go back to its initial configuration. The stress at which the plastic deformation sets in is known as the yield stress of the polymer." According to Das, this is the first study to identify what is happening during the sliding of polymer nanofibers. "We tested fibers with different diameters. Each time, we found that the sliding instability occurred at a particular value of the shear stress—the tangential force divided by the contact size—that is equal to the shear strength of the polymer. This was something we didn't know before, although such a response had been reported before for metals." The study, "Sliding of adhesive nanoscale polymer contacts," was written by Debashish Das and Ioannis Chasiotis. It is published in the Journal of the Mechanics and Physics. Explore further