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Hong Zhao Ph.D.

Associate Professor, Department of Mechanical and Nuclear Engineering, Virginia Commonwealth University

Biotech One, Suite 1085, Richmond, VA, United States


Dr. Zhao’s current research focuses on discovering and understanding new self-assembly mechanisms of colloidal particles.

Zhao pursues the approach of forming well-ordered monolayer structures by assembling colloidal particles at the air–liquid interface, creating a new paradigm in generating coatings and functional devices through inkjet printing.


Mask-assisted electrospray for superoleophobic surfaces: An experimental and numerical study

Surface & Coatings Technology


This paper presents both experimental work and numerical simulations of formation of superoleophobic surfaces created by mask-assisted electrospraying, followed by a second layer overlay and fluoropolymer treatment. The primary electric field focusing in the mask-assisted electrospray effectively guides the electrosprayed particulates into the mesh openings, forming characteristic pyramid-shaped pillars. The secondary focusing occurs during the overlay deposition when the electrosprayed particulates favorably deposit onto the pre-patterned pillars. Systematic studies were conducted on the effects of mask-substrate-gap and duration of the overlay deposition on the pattern morphology and wetting performance. A shorter mask-substrate-gap results in a stronger focusing effect and pillars with a larger aspect ratio. The overlay deposition firstly increases the pillar height and then changes the pillar shape from pyramids to domes with overhangs due to electrostatic interactions. All the surfaces are superhydrophobic, however, superoleophobicity varies. Surfaces that have tall pillars and overhang structures demonstrate robust superoleophobicity when compared to their counterparts with shorter pillars and absence of overhang structures. The primary and secondary electric field focusing effects exerted by the mask and the pre-patterned pillars, and their roles in pattern formation have been numerically investigated by COMSOL Multiphysics simulation. A reasonable agreement has been obtained between the numerical predictions and experimental results.

Pinning and wetting stability of liquids on superoleophobic textured surfaces

Surface Innovations


Super water/oil repellency has been a very active research field and could be achieved via combination of surface texturing and chemical treatment. When a droplet is deposited on a superhydrophobic/superoleophobic surface, its contact line can be pinned somewhere on the structured surface instead of fully wetting the substrate, forming a solid-liquid-air composite interface. The focus of this article is to understand the effect of pinning and wetting stability through energy analysis and force balance. Textured surfaces with straight sidewall pillar, wavy sidewall pillar and hoodoo structures are discussed in detail. For the straight sidewall structure, it was found that the top pillar edge is a stable pinning site for water. Meanwhile, hexadecane fully wets the structure without any pinning. On the wavy sidewall structure, the protruding and concave corners are pinning sites for both water and hexadecane. However, the dominant breakthrough pressure comes from the energy barrier against contact line advancing along the re-entrant slope of the wave. On the hoodoo structure, there are two pinning sites (top and bottom corner of the hoodoo cap) for water, but only one pinning site (bottom corner) for hexadecane. The effects of solid area fraction and re-entrant angles on pinning stability are studied with the wavy sidewall structure. This study suggests that Gibbs energy analysis can be a viable approach in designing robust superoleophobic surfaces by enhancing the pinning stability and breakthrough pressure, which is strongly correlated to design parameters, for example solid area fraction, geometrical re-entrant angle and dimensions. This article contains supporting information that is available online.

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Effect of Surface Texturing on Superoleophobicity, Contact Angle Hysteresis, and Robustness



Previously, we reported the creation of a fluorosilane (FOTS) modified pillar array silicon surface comprising ∼3-μm-diameter pillars (6 μm pitch with ∼7 μm height) that is both superhydrophobic and superoleophobic, with water and hexadecane contact angles exceeding 150° and sliding angles at ∼10° owing to the surface fluorination and the re-entrant structure in the side wall of the pillar. In this work, the effects of surface texturing (pillar size, spacing, and height) on wettability, contact angle hysteresis, and “robustness” are investigated. We study the static, advancing, and receding contact angles, as well as the sliding angles as a function of the solid area fraction. The results reveal that pillar size and pillar spacing have very little effect on the static and advancing contact angles, as they are found to be insensitive to the solid area fraction from 0.04 to ∼0.4 as the pillar diameter varies from 1 to 5 μm and the center-to-center spacing varies from 4.5 to 12 μm. On the other hand, sliding angle, receding contact angle, and contact angle hysteresis are found to be dependent on the solid area fraction. Specifically, receding contact angle decreases and sliding angle and hysteresis increase as the solid area fraction increases. This effect can be attributable to the increase in pinning as the solid area fraction increases. Surface Evolver modeling shows that water wets and pins the pillar surface whereas hexadecane wets the pillar surface and then penetrates into the side wall of the pillar with the contact line pinning underneath the re-entrant structure. Due to the penetration of the hexadecane drop into the pillar structure, the effect on the receding contact angle and hysteresis is larger relative to that of water. This interpretation is supported by studying a series of FOTS pillar array surfaces with varying overhang thickness. With the water drop, the contact line is pinned on the pillar surface and very little overhang thickness effect was observed. On the other hand, the hexadecane drop is shown to wet the pillar surface and the side wall of the overhang. It then pins at the lower edge of the overhang structure. A plot of the thickness of the overhang as a function of the static, advancing, and receding contact angles and sliding angle of hexadecane reveals that static, advancing, and receding contact angles decrease and sliding angle increases as the thickness of the overhang increases.

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Directional Self-cleaning Superoleophobic Surface



In this work, we report the creation of a grooved surface comprising 3 μm grooves (height ∼4 μm) separated by 3 μm from each other on a silicon wafer by photolithography. The grooved surface was then modified chemically with a fluorosilane layer (FOTS). The surface property was studied by both static and dynamic contact angle measurements using water, hexadecane, and a polyethylene wax ink as the probing liquids. Results show that the grooved surface is both superhydrophobic and superoleophobic. Its observed contact angles agree well with the calculated Cassie–Baxter angles. More importantly, we are able to make a replica of the composite wax ink–air interface and study it by SEM. Microscopy results not only show that the droplet of the wax ink “sits” on air in the composite interface but also further reveal that the ink drop actually pins underneath the re-entrant structure in the side wall of the grooved structure. Contact angle measurement results indicate that wetting on the grooved surface is anisotropic. Although liquid drops are found to have lower static and advancing contact angles in the parallel direction, the drops are found to be more mobile, showing smaller hysteresis and lower sliding angles (as compared to the FOTS wafer surface and a comparable 3-μm-diameter pillar array FOTS surface). The enhanced mobility is attributable to the lowering of the resistance against an advancing liquid because 50% of the advancing area is made of a solid strip where the liquid likes to wet. This also implies that the contact line for advancing is no longer smooth but rather is ragged, having the solid strip area leading the wetting and the air strip area trailing behind. This interpretation is supported by imaging the geometry of the contact lines using molten ink drops recovered from the sliding angle experiments in both the parallel and orthogonal directions. Because the grooved surface is mechanically stronger against mechanical abrasion, the self-cleaning effect exhibited in the parallel direction suggests that groove texturing is a viable approach to create mechanically robust, self-cleaning, superoleophobic surfaces.

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Ph.D. Mechanical and Aerospace Engineering Rutgers, The State University of New Jersey 2007

M.S. Mechanical Engineering Petroleum University 2001

B.S. Mechanical Engineering Petroleum University 1996

Area(s) of Expertise

Additive Manufacturing/3d Printing
Printed functional devices
Transport and self-assembly of colloidal nanoparticles
Surface science and surface engineering