Small Droplet, Big Impact:

Multiscale Droplet-Surface Interaction for Water, Environment, and Energy

Youhua's research uniqueness is Multiscale Droplet-Surface Interaction. Based on such, Youhua's career goal is to address problems related with Water, Environment, and Energy. Youhua's research lies at the interface between surface engineering and interfacial phenomena, and hence Youhua's research background has four directions: (1) Surface engineering, (2) Fluid dynamics, (3) Phase-change heat transfer, and (4) physical chemistry. The figure below summarizes Youhua' research experience and future agenda.

1. Surface Engineering

1.1. Fabrication of Micro-/Nano-Textured Surfaces

Nano-/microtextures on surfaces create new surface functions and are essential to the enhancement of many applications. I worked on the fabrication of structures ranging from tens of nanometers to millimeters on silicon-based materials (PDMS and Silicon), carbon-based materials (e.g., SU-8 and polypyrrole), and metallic materials (e.g., Aluminum).

Silicon-based surfaces were textured using standard lithography process. Carbon-based materials were textured using standard lithography too, while the second-order nanostructures were textured using maskless oxygen-plasma etching. Aluminum surfaces were textured using electro-chemical process and hydro-thermal process. The figure below summarizes the fabricated samples.

Related Journal Publications:

1. Junghoon Lee, Youhua Jiang et al., J. Colloid and Interface Sci. 2019.

2. Ke Du, Youhua Jiang et al., Micromachines 2018.

3. Ke Du, Youhua Jiang et al., J. Micromech. Microeng. 2018

4. Deyin Zheng, Youhua Jiang et al., Langmuir 2017.

5. Youhua Jiang et al., Langmuir 2017.

6. Junghoon Lee, Sangwoo Shin, Youhua Jiang et al., Adv. Funct. Mater. 2017.

2. Fluid Dynamics

2.1. Particle-free and Particle-Laden Droplet Motion on Superhydrophobic Surfaces

I investigated the droplet pinning/de-pinning on textured superhydrophobic surfaces. Specifically, I developed models to predict the droplet de-pinning force and contact angles (advancing and receding) on superhydrophobic surfaces, irrespective of surface structure types (pillar vs. pore), dimensions (solid fractions ranging from 0 to 1), directions of contact line motion (advancing vs. receding), or manners in which the droplet been removed from surfaces (laterally sliding vs. vertically detaching).

Related Journal Publications:

1. Donghui Wang, Youhua Jiang et al., Langmuir 2020.
2. Youhua Jiang et al., J. Colloid and Interface Sci. 2019.
3. Mohammad Amin Sarshar*, Youhua Jiang* et al., J. Colloid and Interface Sci. 2019. (co-first author)
4. Youhua Jiang et al., Langmuir 2018.
5. Yujin Sun, Youhua Jiang et al., Surface Innovations 2018.
6. Yujin Sun, Youhua Jiang et al., Surface Innovations 2017.
7. Deyin Zheng, Youhua Jiang et al., Langmuir 2017.
8. Youhua Jiang et al., Progress in Adhesion and Adhesives 2017. (book chapter)
9. Youhua Jiang et al., Review of Adhesion and Adhesives 2016.

2.2. Air-Entrapment of Impacting Droplets on Superhydrophobic Surfaces.

Instead of focusing on the splashing and re-bouncing of impacted droplets on a hydrophobic surface, I investigated the entrapped air layer/bubble underneath the droplet, because it is crucial for applications such as ink-jet printing, anti-icing, pesticide deposition, and coatings. I found that the size/volume of the entrapped air layer is not only determined by the droplet properties and dynamic parameters (e.g., Weber number), but also the droplet-solid interactions (dynamics of contact line).

2.3. Coalescence-Induced Droplet Acceleration along a Superhydrophilic Wire

Droplet coalescence on a plane superhydrophobic surface has been investigated intensively. Droplet along a wire has applications in many fields, such as filtration and mist elimination, but its coalescence and transport received much less attention. In this project, I am investigating an unprecedented droplet transport on superhydrophilic wires that suddenly and significantly increases the droplet speed. This spontaneous self-acceleration is powered by the surface-to-kinetic energy transition that occurs upon coalescence, exceeding the contribution from the potential energy. The energy transition is due to the speed difference of two wedges of the coalesced droplet in damped oscillation and its efficiency is determined by the viscous friction.

Related Journal Publications:

1. Youhua Jiang et al., Phys. Rev. Lett. (under review)

Related presentations:

1. O’Donnell, A.; Jiang, Y.; Park, K.-C., Droplet Propulsion on a Superhydrophilic Wire Induced by Coalescence. 72^nd APS Division of Fluid Dynamics, November 23-26, 2019, Seattle, Washington, USA.

2.4. Micro-scale Droplets Impact onto a Wire (Fog Collection)

One of the most promising approaches to obtain additional potable water at a low cost is collecting water from the fog-laden wind or industrial processing plants (e.g., cooling towers). Such sources of suspended liquid droplets are found prevalent in the world. The process of capturing fog droplets is fundamentally an aerodynamics-determined process and hence aerodynamic system parameters should be analyzed. In this project, I systematically investigated how the onset time of fog collection and fog collection efficiency are determined by the deposition efficiency (an aerodynamic parameter) and droplet retention force (an interfacial phenomena parameter).

Related Journal Publications:

1. Youhua Jiang et al., Soft Matter 2019. (Back Cover)

2. Youhua Jiang et al., Applied Physics Letters 2019. (Highlighted in Nature Materials - "Drinking up mist")

Related presentations:

1. Jiang, Y.; Machado, C.; Savarirayan, S.; Patankar, N.A.; Park, K.-C., Onset Time of Fog Collection Using a Single Wire. 72^nd APS Division of Fluid Dynamics, November 23-26, 2019, Seattle, Washington, USA.
2. Jiang, Y.; Machado, C.; Savarirayan, S.; Patankar, N.A.; Park, K.-C., Onset Time of Fog Collection. 1^st International Conference on Nature Inspired Surface Engineering, June 11-14, 2019, Hoboken, New Jersey, USA.
3. Jiang, Y.; Machado, C.; Savarirayan, S.; Park, K-C., Fog Harvesting on Wire Arrays. APS March Meeting, March 4-8, 2019, Boston, Massachusetts, USA.
4. Jiang, Y.; Savarirayan, S.; Yao, Y.; Park, K-C., Fog Harvesting on a Vertical Wire. 71th APS Division of Fluid Dynamics, November 18-20, 2018, Atlanta, Georgia, USA.

2.5. Oil-Impregnated Oxide Nanostructures for Durable Slippery Surfaces.

The impregnated oil layer significantly enhances the surface functionality for various applications, such as condensation, anti-icing, self-cleaning, anti-corrosion, and anti-fouling. However, the depletion of the oil layer compromises the surface durability. Thus, I investigated the rationale by which the surface should be engineered to efficiently retain the oil layer, especially under harsh environment. I used anodic aluminum oxide surfaces with high-aspect-ratio nanopores hydrophobilized with Teflon to demonstrate long-term oil retention.

Related Journal Publications:

1. Junghoon Lee, Youhua Jiang et al., J. Colloid and Interface Sci. 2019.

2. Junghoon Lee, Sangwoo Shin, Youhua Jiang et al., Adv. Funct. Mater. 2017.

3. Phase-Change Heat Transfer

3.1. Structured Surfaces for the Decrease of Ice Adhesion.

The effects of surface properties on de-icing performance has been examined. The forces required to laterally de-pin the frozen droplets have been quantified.

3.2. Self-Detaching Ice Layer on Extremely Cold Surfaces

When a droplet impacts onto an extremely cold superhydrophilic surface, the droplet immediately spreads out and forms an ice layer. Due to the sudden formation of ice, an internal stress builds up. Such a stress leads to a spontaneous cracking of the ice layer, so that the ice-surface adhesion force can be significantly reduced.

Related Journal Publications:

1. Dong Song, Youhua Jiang et al., submitted.

3.3. Condensation and Ice Formation on Textured Surfaces Using ESEM

Condensation and ice formation are affected by surface chemical and physical properties. In this project, I investigated the condensation and ice formation on textured superhydrophobic surfaces using Environmental Scanning Electron Microscope.

4. Physical Chemistry

4.1. Droplet Manipulation on Stimuli-Responsive Conjugated Polymer Surfaces.

Regarding the manipulation/actuation of droplets/liquids on a responsive surface [PPy(DBS) surface in my study], the response time, which is defined as the period from applying external stimuli to the onset of droplet motion, needs to be minimized for applications such as microfluidics, water treatment, and bio-detections. I investigated the combined effect of the droplet wetting state and the release rate of doped surfactants, both of which are affected by surface structure dimensions.

Related Journal Publications:

1. Wei Xu, Anthony Palumbo, Jian Xu, Youhua Jiang et al., ACS. Appl. Mater. Interfaces 2017.

2. Youhua Jiang et al., Langmuir 2017.

Related Conference Presentations:

1. Youhua Jiang et al., 91th Colloids & Surface Science Symposium 2017, New York City, New York, USA.

4.2. Site-Specified Self-Assembly of Particles on Superhydrophobic Surfaces.

The main objective is to assemble micro-/nano- particles on specified locations (e.g., pillar tips) by taking the advantage of the discontinuous droplet contact line on pillar-structured surfaces. Unique to pillar-structured surfaces, the contact line recedes in a discontinuous manner, where capillary bridges form and rupture in between the bulk receding droplet and the liquids (containing particles) remained on pillar tips. By allowing droplets containing particles with different properties (e.g., positively vs. negatively charged) successively evaporate on the same location, multi-layers of particles can be deposited on specified locations.

Related Journal Publications:

1. Youhua Jiang et al., submitted.

Related Conference Presentations:

1. Jiang, Y.; Xu, W.; Connington, K.; Choi, C.-H., Effects of Nanoparticles on the Depinning Force of a Receding Droplet on Micropatterned Superhydrophobic Surfaces. 9th ICMF international conference on multiphase flow, May 22-27, 2016, Firenze, Italy.