Alexander von Humboldt Research Fellow Professor Chang-Hwan Choi

Research Fellow – Facts

Professor Dr. Chang-Hwan Choi. Photo: Claus Völker
Prof. Dr. Chang-Hwan Choi. Photo: Claus Völker

Name: Professor Dr. Chang-Hwan Choi

Age: 43

Area of research:
From Microfluidics to Nanofluidics

Name of my University / Research Institute: Stevens Institute of Technology, Hoboken, New Jersey, USA

Research period at the TU Darmstadt:
February 1 to April 30, 2016


Questionnaire to the Research Fellow

My field of research is fascinating. To laymen I would explain it in the following comprehensible manner:
Water flow through nanoscale channels can create electric power called streaming current. However, such nanoscale hydrodynamic flows have not yet been intensively investigated for practical energy harvesting systems, particularly due to their low energy conversion efficiency resulting from significant frictional energy loss at the channel walls. My research on nanofluidics is to understand fundamentals on such nanoscale hydrodynamic flows and lay the technical foundation for the development of nanofluidic energy harvesting systems with high hydroelectric energy conversion efficiency and output power that are meaningful for real applications. Working as a “hydropower plant on a chip”, the nanofluidic energy harvesting systems can power small autonomous sensors, their networks, and mobile/wearable devices without batteries. Scaled-up systems (e.g., arrays of such devices) can further serve as reliable power stations for large systems with amplified power output. Scavenging energy from water, a sustainable/renewable resource, will enable such systems to be functional with little influence by ambient conditions.

My most important success in research to date is…
Working at Stevens Institute of Technology since 2007, my research perspectives have focused on the fundamental understanding of nanoscale interfacial phenomena and the development of scalable nanomanufacturing techniques that can allow the scientific studies of nanomechanics and the broad applications of nanostructures, ultimately targeted for the engineering of multifunctional/adaptable surfaces, devices, and systems for multiscale civil, military, energy applications. Supported by various funding sources including NSF, ONR, DARPA, US Army, DOE, ACS PRF, USDA, and Industry, I have developed 17 research grants/contracts with a value greater than $5M during my time at Stevens. In total, 5 post-docs, 9 PhD students, and 7 MS students have been trained while working on these projects. The research projects have resulted in significant scholarly achievements including more than 100 peer-reviewed journal and conference articles (total number of citation greater than 2600, h-index: 22, and i10-index: 30), 4 patents issued/pending, and more than 100 presentations at premium conferences, workshops, symposiums, universities and national labs including 4 keynote talks.

Several of the research activities have also been highlighted in the news media, including Nature (“Fluid dynamics: Slip and slide”, Nature 454, 920, 2008). I have also received several awards for outstanding research achievement, including the ONR Young Investigator Award, Stevens Research Recognition Award, and NSF Fellowships. In particular, the ONR Young Investigator Award was also highlighted in Nature (“From Aerospace to Navy Ships: Design for Anti-Corrosive Vessel Surfaces Earns Award for Nanoengineer”, Nature 465, 385, 2010. I was also selected as one of Nanotechnology Thought Leaders in 2010 by AZoNano. I was designated one of the eight US delegates to attend the CRDF Global Workshop in Uzbekistan to present and discuss the developments in energy research and collaboration between the US and Uzbekistan.

One of my major research achievements is in the innovation of nanomanufacturing systems and technologies, especially for large-area 3D nanopatterning and nanostructure fabrication. For full wafer-scale (and beyond), high-rate and low-cost nanolithography techniques capable of precise control of the three-dimensionality of nanostructures, tunable interference lithography systems and processes have been developed by my group. One of the systems has been issued for an US patent and recognized by the New Jersey Inventors Hall of Fame Award. Using the wellregulated large-area periodic nanostructures as physical templates, simple pattern transfer methods for various types of thin films and nanostructures (e.g., metals and graphene) to different substrate materials including transparent and flexible substrates (e.g., glass, PDMS, and PMMA) have also been developed, which would be of great significance in many applications such as microfluidics, sensors, and wearable electronics. The nanostructured templates have also enabled site-specific layerby-layer self-assembly of nanomaterials (e.g., nanowires, nanofibers, and nanochannels) with controlled material compositions, anisotropy, and hierarchy via physical/chemical vapor depositions as well as simple atmospheric evaporative processes. Such composite nanostructures will significantly enhance our capability to realize future smart materials and advanced systems such as high-efficiency energy harvesting materials, nanoelectronics, nanophotonics, optical/magnetic devices and sensors.

As opposed to conventional stencil lithography techniques using “hard” masks, a new “soft” stencil lithography technique has also been developed using the lithographically patterned photoresist (polymer) films as free-standing flexible membranes. Used as free-standing soft appliqué, the nanotextured flexible membrane has enabled the nanopatterning of non-planar and curved surfaces such as cylinders, spheres, trenches. Exploiting self-masking effects in oxygen plasma etching processes, a simple maskless fabrication process for high-aspect-ratio polymer nanowires has also been developed. Novel 3D anodizing techniques have also been developed, for the first time, capable of hybrid and hierarchical nanostructuring of metallic substrates. Anodization has widely been used to produce self-ordered nanostructures over a large surface area of various metallic substrates. However, the current anodizing technology normally forms only 2D planar porous structures, which seriously limit the efficacy and applicability of metallic nanostructures in many civil/military applications. The 3D anodizing processes developed in my group have opened up a new way to realize 3D hybrid and hierarchical nanostructures via simple, convenient, and scalable electrochemical etching processes, which will significantly advance the broader applications of metallic nanostructures with immediate impacts to manufacturing industries.

Another significant scientific contribution I have made is in surface/interfacial science and phenomena such as wetting, friction, adhesion, and energy transfer. In particular, enabled by the large-area 3D nanopatterning systems and techniques developed my group as described above, new fundamental understanding of novel interfacial phenomena at the nanoscale, which were unexplored previously due to the lack of experimental precision, has been established, including superhydrophobicity, contact angle hysteresis, tunable wettability of polymers, hydrodynamic slip and friction, phase-chase heat transfer processes such as evaporation, condensation, boiling, and icing, corrosion, and microbiological cell adhesions. One of the highlights in this area is that new physical insight on adhesion and friction of liquid drops (e.g., contact angle hysteresis) has been revealed, which is of great importance in numerous applications such as coating/spraying, microfluidics, thermal/energy systems, and biotechnology.

Traditionally, it had been believed that the interfacial contact area is a key parameter to determine the adhesion and friction properties of droplets. However, our work has discovered that the dynamics of a three-phase contact line is a more direct factor to govern the adhesion and friction properties than the kinetic effect of the area. Such new discovery has further enabled a better understanding and control of broader interfacial phenomena such as tunable wettability in redox processes of conjugated (conducting) polymers and pH-dependent layer-by-layer coatings, kinetics and dynamics in mass/energy transfer processes of droplets/bubbles such as evaporation, fogging/condensation, boiling, and frosting/icing. Another of our scientific breakthrough is the new understanding of wall slip flows and its engineering application to hydrodynamic friction reduction. Conventionally, a noslip boundary condition at a solid wall had been adopted in most viscous flows, predicting the macroscale hydrodynamic behaviors very well. However, it has been discovered that the no-slip boundary condition should break down in micro and nanoscales, significantly influenced by surface roughness and wettability. Moreover, it has been shown that slip can be regulated by engineering the surface micro/nano-textures, increasing linearly with pattern periodicity and exponentially with air fraction in case of superhydrophobic surfaces.

I have also demonstrated, for the first time, that such a giant slip can reduce hydrodynamic friction significantly, not only in laminar flow but also in high Reynolds number turbulent flow, which will result in great energy saving and energy conversion efficiency in many fluid engineering systems. It has also been shown that such slippery superhydrophobic surfaces prevent surfaces from corrosion and biofouling, having truly energy-efficient, multifunctional and broad applicability.

I’ve chosen the TU Darmstadt because of…
My host, Prof. Steffen Hardt is one of the world-leading experts in the microfluidics and nanofluidics researches. Especially, his group already developed the theoretical model and foundation on the electrohydrodynamics in superhydrophobic channels, which is critical in my proposed research as well. Thus, I have chosen him and TU Darmstadt to learn from and collaborate with. The Center of Smart Interfaces at the TU Darmstadt is also known as one of the leading centers studying many interfacial phenomena of fluids, which is also one my research interests in general. Thus, I was also very interested in learning the broad research activities at the Center of Smart Interfaces at the TU Darmstadt, while working with Prof. Hardt.

Host Prof. Dr. Steffen Hardt with Prof. Dr. Chang-Hwan Choi. Photo: Claus Völker
Host Prof. Dr. Steffen Hardt with Prof. Dr. Chang-Hwan Choi. Photo: Claus Völker

With the help of my host in Darmstadt I would like to…
The ultimate goal of my research is to verify the theoretical prediction that a superhydrophobic surface reducing viscous wall friction with significant slip can increase ionic streaming current in nanoscale channel flow and consequently the electrokinetic energy conversion efficiency and also the output power. To achieve this objective, with the help of my host (Prof. Steffen Hardt) in Darmstadt, I would like to conduct a theoretical and numerical analysis for the electrostatically and hydrodynamically heterogeneous boundary conditions configurable on various types of superhydrophobic surfaces. The resulting streaming current and flow properties will be computed by coupling the Poisson-Boltzmann and the Navier-Stokes equations and employing the fluidic circuitry based on Onsager reciprocal relations. The theoretical results will then be verified with nanofluidic experiments by testing superhydrophobic nanochannels of regulated sizes for varying electrolytes and flow conditions in the future.


Questionnaire for the host

Prof. Dr. Steffen Hardt. Photo: Claus Völker
Prof. Dr. Steffen Hardt. Photo: Claus Völker

Guest of: : Prof. Dr. Steffen Hardt and Prof. Dr. Hans-Jürgen Butt

Department: Mechanical Engineering, TU Darmstadt and Max-Planck-Institut für Polymerforschung, Physik der Grenzflächen

You appreciate in your guest / your guest favourably impressed you by…
his pioneering work in the field of superhydrophobic surfaces. He was among the first to show that with tailor-made superhydrophobic surfaces an enormous drag reduction of hydrodynamic flow can be achieved.

You, your team and the TU Darmstadt benefit from your guest’s…
long-standing experience in the design, fabrication and experimental characterization of superhydrophobic surfaces. At the Institute for Nano- and Microfluidics we also work on superhydrophobic surfaces, but so far our focus was mainly on the theoretical modeling of flow along such surfaces. The work of Professor Choi nicely complements our activities.