Tunneling of Water Droplets Through a Liquid Thin Film Xiao-Lun Wu, University of Pittsburgh, DMR The required kinetic energy of an incoming particle on the penetration of another material of thickness h is strongly dependent on the physical process. Bombardment of metal by accelerated ion particles has been studied for many years and led to the fundamental understanding of the atomic structures. The energy and thickness dependence scale as h~E 2. The situation appears to be significantly different for ballistic projectiles, such as a bullet and an armor piercing rocket. In this case, the resistance force generated by the target is nearly constant, yielding the well known result h~E. We investigated the physical process of penetration of a thin liquid film by a micron-sized ballistic water droplet. We found an interesting threshold behavior, i.e. a finite amount of energy is required even when h→0. The study is useful for a variety of industrial processes such as droplet encapsulation and gene transmission through biological tissues (Kim and Wu, Phys. Rev. E 82, (2010)). Figures (a-d) depict a tunneling process where the incoming particle is highlighted by the solid arrow and its mirror image is highlighted by the dotted arrow. The process is imaged by a video system sketched in (e). The initial penetration creates a pouch of ~100  m in length (see the blowup image in (a)) and then leaves behind a hole as seen in (b). The hole anneals rapidly, within one video frame or less than 0.25 millisecond and not visible in (c).

* Teaching and training research scientists have always been our highest priority. The training takes place via normal lectures and practice in the laboratory. The training also occurs at different levels as symbolized the picture to the left. Here Roy Cerbus is posed along with the PI (left) and Dr. Sudo Chattopadhyay (right). Roy was an undergraduate from Purdue and did a summer intern via an REU program in our lab. He is now a physics graduate student in our department. Sudo on the other hand, just completed his Ph.D. degree and is now a research scientist at Intel Co. My other graduate student Ildoo Kim will be getting his Ph.D. degree shortly. * I also participated in training local high- school students using the opportunities of our departmental open houses. I gave lectures in optics and fluid dynamics. These students also participated in hand- on physics demonstrations. Please insert an image or group of images here to illustrate your broader impacts activities. If you need more space, you may reduce the adjacent textbox. Please use lettering that is clearly visible (i.e. not too small). Please include a brief figure caption. * My student Ildoo Kim and I have been working with our local medical researchers to evaluate bio safety of their flow cytometers. These machines shoot out small water droplets with biological cells enclosed. Some of those cells are harmful to environment, and their containment is essential. Using our physics knowledge and equipment, we made the evaluation scientifically rigorous. Tunneling of Water Droplets Through a Liquid Thin Film Xiao-Lun Wu, University of Pittsburgh, DMR