Zhichao Dong

Manipulating and dispensing micro/nanoliter droplets by superhydrophobic needle nozzles.

There is rapidly increasing research interest focused on manipulating and dispensing tiny droplets in nanotechnology and biotechnology. A micro/nanostructured superhydrophobic nozzle surface is one promising candidate for the realization of tiny droplet manipulating applications. Here, we explore the feasibility of using superhydrophobicity for guided dispensing of tiny water droplets. A facile dip-coating method is developed to prepare superhydrophobic needle nozzles (SNNs) based on commercial needle nozzles with reduced inner diameter. The SNNs can manipulate tiny droplets of different volumes by only changing the inner diameter of the nozzle, rather than reducing the nozzle size as a whole. Different from the previous electric-field-directed process or pyroelectrodynamic-driven technique, quasi-stable water drops down to the picoliter scale can be produced by SNNs without employing any extra driving mechanisms. Due to their intrinsic superhydrophobic nature, the SNNs also possess the properties of reducing sample liquid retention, improving sample volume transfer accuracy, and saving expensive reagents. In addition, this kind of dip-coating method can also be applied to micropipet tips, inkjet or bio-printer heads, etc. As the issues of reducing drop size and increasing drop volume accuracy are quite important in the laboratory and industry, this facile but effective superhydrophobic nozzle-coating method for manipulating tiny droplets could be of great help to make breakthroughs in next-generation liquid transport and biometric and inkjet printing devices.

Controlling liquid splash on superhydrophobic surfaces by a vesicle surfactant

By adding a small amount of a vesicle surfactant, “unavoidable” splashing is considerably reduced on superhydrophobic surfaces. Deposition of liquid droplets on solid surfaces is of great importance to many fundamental scientific principles and technological applications, such as spraying, coating, and printing. For example, during the process of pesticide spraying, more than 50% of agrochemicals are lost because of the undesired bouncing and splashing behaviors on hydrophobic or superhydrophobic leaves. We show that this kind of splashing on superhydrophobic surfaces can be greatly inhibited by adding a small amount of a vesicular surfactant, Aerosol OT. Rather than reducing splashing by increasing the viscosity via polymer additives, the vesicular surfactant confines the motion of liquid with the help of wettability transition and thus inhibits the splash. Significantly, the vesicular surfactant exhibits a distinguished ability to alter the surface wettability during the first inertial spreading stage of ~2 ms because of its dense aggregates at the air/water interface. A comprehensive model proposed by this idea could help in understanding the complex interfacial interactions at the solid/liquid/air interface.

Uni-Directional Transportation on Peristome-Mimetic Surfaces for Completely Wetting Liquids

Liquid uni-directional transport on solid surface without energy input would advance a variety of applications, such as in bio-fluidic devices, self-lubrication, and high-resolution printing. Inspired by the liquid uni-directional transportation on the peristome surface of Nepenthes alata, here, we fabricated a peristome-mimicking surface through high-resolution stereo-lithography and demonstrated the detailed uni-directional transportation mechanism from a micro-scaled view visualized through X-ray microscopy. Significantly, an overflow-controlled liquid uni-directional transportation mechanism is proposed and demonstrated. Unlike the canonical predictions for completely wetting liquids spreading symmetrically on a high-energy surface, liquids with varied surface tensions and viscosities can spontaneously propagate in a single preferred direction and pin in all others. The fundamental understanding gained from this robust system enabled us to tailor advanced micro-computerized tomography scanning and stereo-lithography fabrication to mimic natural creatures and construct a wide variety of fluidic machines out of traditional materials.

Superwettability Controlled Overflow

Superwettability controlled overflow is presented. Superhydrophilicity enhances overflow and superhydrophobicity reduces overflow. The fundamental mechanism for the dynamic interaction between fluids and solid-edges is revealed and several methods for preparing the surfaces of solid-edges with superwettabilities are presented. The insights gained provide new opportunities to achieve controllable dynamic interaction at solid-liquid interfaces for various applications.

Manipulating Overflow Separation Directions by Wettability Boundary Positions.

Facile strategies to realize controllable overflow separation are urgently needed for advances in liquid-directional transportation systems and liquid delivery devices. Here, we present a wettability boundary based destabilization mechanism for direct separation of liquid flow from the solid edge at the (super)hydrophilic-superhydrophobic dividing line. Macroscale fluid dynamics is precisely controlled by modifying micro- and nanoscale surface structures and chemical compositions. Coupling surface wettability boundaries with flow inertia, flow separation angles are finely adjusted. These findings not only provide physicochemical insight into the understanding of the mechanisms on the dynamics of fluid at solid edges, but also promote the development of nanoscience in hydrodynamic applications.

Manipulating Oil Droplets by Superamphiphobic Nozzle

will be reduced and liquid retention on such nozzles is to be avoided, which is of great importance to the development of tiny oil droplets fabrication. We have prepared a superhydrophobic needle nozzle to manipulate water droplets. [ 13 ] However, most functional materials for fabrication of organic solar cells, organic fi eld effect transistors, and organic light-emitting diodes are only soluble in organic liquids with low surface tensions. As organic liquid will spread and wet the water repellent surface, the superhydrophobic nozzle is not able to manipulate such low surface tension liquids. Thus, it is still a challenge to increase the liquid transfer effi ciency of organic liquid. Here, we report a facile strategy to manipulate small organic liquid droplets assisted by a superamphiphobic nozzle. Owing to the superliquid-repellent property of the nozzle surface, oil droplet is restricted at the edge of the nozzle, which avoids the spreading of oil droplet and results in decreased volume as well as increased liquid transfer effi ciency. Furthermore, by utilizing the superliquid-repellent nozzle to directly dispense oil-based inks, high-resolution 3D structures are fabricated, which is of great signifi cance for the development of liquid transportation and ink-jet printing devices. Figure 1 a shows the preparation process of the hierarchically structured superamphiphobic nozzle surface. Copper plates (5 × 5 × 1 mm 3 ) were used as the base material for nozzle fabrication, and holes were drilled through the plates for tiny oil droplet dispensing. Holes with diameters ranged from 20 to 150 μm were prepared by laser drilling (Figure 1 b), and holes larger than 150 μm were fabricated by mechanical drilling (Figure 1 c). To modify the nozzle surface with reentrant micronanostructures, chemical base corrosion was performed, [ 14 ] in which Cu plates were immersed in an ammonium persulfate and sodium hydroxide mixture solution (details about the morphology and wettability of different corrosion time can be seen in Figures S1 and S2, Supporting Information). As shown in Figure 1 d, Cu(OH) 2 microsheets and microclusters with re-entrant structures are achieved. Since surface wettability is controlled by both surface morphology and surface chemical composition, [ 15 ] only a structured route is not enough to realize superamphiphobicity on the nozzle surface. Fluorination was thus used to lower the surface energy. X-ray photoelectron spectroscopy (XPS) spectra demonstrated the successful introduction of fl uorine element on the prepared surface and the conversion of Cu(OH) 2 to Cu(O 2 C 2 H 2 C 8 F 17 ) 2 (Figure S3, Supporting Information). In addition, scanning electron microscope (SEM) images showed that almost no visual morphology change occurred after fl uorination (Figure S4, Supporting Information). DOI: 10.1002/smll.201501021 Oil Droplets

Ballistic Jumping Drops on Superhydrophobic Surfaces via Electrostatic Manipulation

The ballistic ejection of liquid drops by electrostatic manipulating has both fundamental and practical implications, from raindrops in thunderclouds to self-cleaning, anti-icing, condensation, and heat transfer enhancements. In this paper, the ballistic jumping behavior of liquid drops from a superhydrophobic surface is investigated. Powered by the repulsion of the same kind of charges, water drops can jump from the surface. The electrostatic acting time for the jumping of a microliter supercooled drop only takes several milliseconds, even shorter than the time for icing. In addition, one can control the ballistic jumping direction precisely by the relative position above the electrostatic field. The approach offers a facile method that can be used to manipulate the ballistic drop jumping via an electrostatic field, opening the possibility of energy efficient drop detaching techniques in various applications.

Surfaces Inspired by the Nepenthes Peristome for Unidirectional Liquid Transport

The slippery peristome of the pitcher plant Nepenthes has attracted much attention due to its unique function for preying on insects. Recent findings on the peristome surface of Nepenthes alata demonstrate a fast and continuous unidirectional liquid transport, which is enabled by the combination of a pinning effect at the sharp edges and a capillary rise in the wedge, deriving from the multiscale structure, which provides inspiration for designing and fabricating functional surfaces for unidirectional liquid transport. Developments in the fabrication methods of peristome-inspired surfaces and control methods for liquid transport are summarized. Both potential applications in the fields of microfluidic devices, biomedicine, and mechanical engineering and directions for further research in the future are discussed.

Janus Gradient Meshes for Continuous Separation and Collection of Flowing Oils under Water

Gradient meshes with Janus wettabilities are fabricated to stably separate and collect spilled oils from a range of flowing oily wastewater. Here, we demonstrate an overflow with separation methodology, which combines selective oil overflow and membrane separation, to separate low content oils from dynamic flowing oil-water mixtures by a curved gradient mesh that covered on a solid edge. The microscaled air-oil-water-solid four-phase wetting state during the oil-water separation process is visualized and demonstrated. The fundamental understanding of this overflow with separation system and the superior gradient mesh materials would enable us to construct a wide variety of separation devices out of traditional designs and advance related applications, such as wastewater treatment and fuel purification.

Time-Dependent Liquid Transport on a Biomimetic Topological Surface

Liquid drops impacting on a solid surface is a familiar phenomenon. On rainy days, it is quite important for leaves to drain off impacting raindrops. Water can bounce off or flow down a water-repellent leaf easily, but with difficulty on a hydrophilic leaf. Here, we show an interesting phenomenon in which impacting drops on the hydrophilic pitcher rim of Nepenthes alata can spread outward to prohibit water filling the pitcher tank. We mimic the peristome surface through a designed 3D printing and replicating way and report a time-dependently switchable liquid transport based on biomimetic topological structures, where surface curvature can work synergistically with the surface microtextures to manipulate the switchable spreading performance. Motived by this strange behavior, we construct a large-scaled peristome-mimetic surface in a 3D profile, demonstrating the ability to reduce the need to mop or to squeegee drops that form during the drop impacting process on pipes or other curved surfaces in food processing, moisture transfer, heat management, etc.