Miljkovic group develops improved technique to optically image dynamic droplet processes

 

Research led by MechSE Assistant Professor Nenad Miljkovic and published in the American Chemical Society’s journal ACS Nano illustrates the development of a single-camera technique capable of providing 3D information on droplet-surface interactions through the use of focal plane manipulation. 

 

Termed focal plane shift imaging (FPSI), Miljkovic utilized the technique to study the spontaneous droplet jumping process on superhydrophobic surfaces—which is governed by inertial-capillary energy conversion—with a wide range of structure length scales and droplet radii. 

 

Droplet-surface interactions are common in many natural and industrial processes thanks to their large surface-to-volume ratios and ability to quickly exchange energy, mass, and momentum. 

 

To study droplet jumping, scientists historically have used an orthogonal two-camera setup to observe the out-of-plane droplet motion, but because of the inability to obtain initial droplet radii and jumping velocity, this procedure has its limitations.   

 

In their paper, “Focal Plane Shift Imaging for the Analysis of Dynamic Wetting Processes,” Miljkovic and his team benchmarked the FPSI technique and studied the effects of droplet mismatch, multidroplet coalescence, and multihop coalescence on droplet jumping speed. 

 

“We were able to resolve the full trajectory of multiple jumping events, to show that, unlike previously theorized, the departure angle during droplet jumping is not a function of droplet mismatch or number of droplets coalescing prior to jumping. Rather, angular deviation arises due to in-plane motion post coalescence governed by droplet pinning,” said Miljkovic. 

 

Their findings offer insight into a droplet’s departure angle; how to rationally design surfaces to control the jumping speed and direction; and whether dynamic wetting processes can be used to improve the performance of technologies that utilize droplet motion. 

 

The knowledge gained from this work can be applied to the development of more compact condensers for HVAC and electrical power generation applications, better designed anti-icing surfaces, or highly efficient electronics thermal management devices. Tailoring of the superhydrophobic surface design to enhance droplet jumping increases the condensation heat transfer. With this newly developed FPSI technique, researchers now have a direct view of the jumping process to help them achieve next-generation surfaces. Miljkovic said he is excited to use the technique to study other droplet-droplet or droplet-surface interactions such as coalescence, sliding, and impact. 

 

Colleagues in Miljkovic’s field of research, including Jonathan Boreyko, an assistant professor of biomedical engineering and mechanics at Virginia Tech, and Konrad Rykaczewski, assistant professor at the School for Engineering of Matter, Transport and Energy at Arizona State University, communicated the appeal of Miljkovic’s new technique. 

 

“Since its discovery in 2009, jumping-droplet condensation has been very hard to characterize due to the unpredictable three-dimensional trajectories of the microscopic jumping droplets,” said Boreyko. “Professor Miljkovic’s group invented a very clever technique for imaging the 3D motion of jumping droplets using only a single camera and top-down microscope. By intentionally placing the focal plane of the microscope a known distance away from the condensing surface, the jumping droplets come into focus in midair, making it possible to reconstruct their 3D trajectory. This novel technique has already led to new insights regarding the dynamics of droplet jumping, which will hopefully lead to improved jumping-droplet condensers in the future that could be used for enhanced heat transfer, anti-icing, or self-cleaning applications.”

 

“His group’s latest paper introduces a cleverly simple, yet powerful, optical technique to characterize motion of droplets ejecting upon coalescence during water condensation on superhydrophobic surfaces,” said Rykaczewski. “The technique essentially consists of taking an initial picture of drops at the surface then focusing a bit above the drops, and waiting for the jumping drop to come into and out of the ‘new focus.’ In contrast to many other advanced techniques for characterization of this process, like Environmental SEM, the equipment used for focal plane shifting imaging is commonly available, which will increase its impact. Professor Miljkovic’s group demonstrated the utility of this technique by using it to uncover previously unknown aspects of the jumping droplet mechanisms, including the influence of the initial droplet size on the jumping direction. Previously we thought coalescence of drops with mismatched diameters would lead to, as the authors nicely put it, ‘rise of angular deviation’ (i.e. a crooked jump). Instead, it is pinning of the ‘product’ droplet after coalescence that leads to this interesting behavior.” 

 

The paper was co-authored by several graduate students in Miljkovic’s group, Hyeongyun Cha (BSME ’14) and Jesus Sotelo (BSME ’14), and MechSE undergraduate Jae Min Chun.

 

“The real credit should go to my students. Although we all developed the initial idea, they are the ones who executed the research flawlessly,” said Miljkovic, who also recently received a New Investigator Award from the American Chemical Society. 

 

Miljkovic received his BS in mechanical engineering from the University of Waterloo in 2009, and an MS and PhD in mechanical engineering from MIT in 2011 and 2013. He joined MechSE in 2014. 

 

In his Energy Transport Research Lab, Miljkovic’s research spans the fields of thermo-fluid sciences, interfacial phenomena, and renewable energy. He aims to bring about transformational efficiency enhancements in energy, water, agriculture, transportation, and electronics cooling by fundamentally manipulating heat-fluid-surface interactions across multiple length and time scales.