Cunlong Yu

Superhydrophobic helix: controllable and directional bubble transport in an aqueous environment

Manipulating air bubbles in an aqueous medium exhibits both scientific and technologic value in gas-collection, selective aeration, and pollutant disposal. Superhydrophobic substrates, known as underwater superaerophilic substrates, offer numerous opportunities to develop advanced gas controlling systems, arising from its strong affinity to air bubbles in water. Herein, we present a superhydrophobic helix that is able to achieve controllable and directional bubble delivery. In an aqueous environment, the bubble tends to stay on the summit of the helix structure and moves along with the helix rotation. The velocity of the bubble delivery can be facilely tuned in terms of the spacing length of the helix. Continuous bubble collection and delivery were realized by integrating the helix with a gas needle and anti-buoyancy transport of the air bubbles was demonstrated using a tilted superhydrophobic helix. Taking advantage of the bubble controllability, a bubble based micro-reaction of H2 and O2 was conducted depending on the special helix structure with two directionalities. This contribution should provide new ideas for the exploration of functional superwettability materials and promote the development of gas-involved multi-phase systems.

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.

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.

Drop Cargo Transfer via Unidirectional Lubricant Spreading on Peristome-Mimetic Surface

To promote drop mobility, lubricating the gap between liquid drop and solid surface is a facile method which has been widely exploited by nature. Examples include lotus and rice leaves using entrapped air to lubricate water and Nepenthes pitcher plant using a slippery water layer to trap insects. Inspired by these, here, we report a strategy for transporting drop cargoes via the unidirectional spreading of immiscible lubricants on the peristome-mimetic surface. Oleophilic/hydrophobic peristome-mimetic surfaces were fabricated through replicating three-dimensional printed samples. The peristome-mimetic surface, via unidirectional immiscible hexadecane spreading, can transport a wide diversity of drop cargoes over a long distance with no loss with controllable drop volumes and velocities, hence mixing multiphase liquids and even reacting liquids. We anticipate this unidirectional drop cargo transport technique will find use in microfluidics, microreactors, water harvesting systems, etc.

Liquids Unidirectional Transport on Dual-Scale Arrays

Liquids unidirectional transport has cutting-edge applications ranging from fog collection, oil-water separation, to microfluidic devices. Despite extensive progresses, existing man-made surfaces with asymmetric wettability or micro/nanoscales structures are still limited by complex fabrication techniques or obscure essential transport mechanisms to achieve unidirectional transport with both high speeds and large volumes. Here, we demonstrate the three-dimensional printed micro/macro dual-scale arrays for rapid, spontaneous, and continuous unidirectional transport. We reveal the essential directional transport mechanism via a Laplace pressure driven theory. The relationship between liquid unidirectional transport and surface morphology parameter is systematically explored. Threshold values to achieve unidirectional transport are determined. Significantly, dual-scale arrays even facilitate liquids uphill running, microfluidics patterning, and liquid shunting in target directions without external energy input. Free combination of dual-scale island arrays modules, just like LEGO bricks, achieves fast liquid transport on demand. This dual-scale island array can be used to build smart laboratory-on-a-chip devices, printable microfluidic integration systems, and advanced biochemistry microreactors.