Xingguo Geng

Impact dynamics of droplets with silica nanoparticles and polymer additives

We investigate the impact dynamics of droplets containing silica nanoparticles and/or poly(ethylene oxide) (PEO) additives by using a high speed camera, and relate the impact behavior to the rheological properties of liquids. For a droplet with both particles and polymer additives, the rebound is damped much faster and the instability behavior is suppressed. Interestingly, the rebound can be inhibited even when impact is at high velocity (1.88 m s−1). The transition from “rebound” to “stick” by enhancing the impact velocity is mainly due to the increase of the friction force of the nanoparticles and polymer aggregates with the substrate. This is confirmed by the increase of the sliding angle with impact velocity.

Switchable opening and closing of a liquid marble via ultrasonic levitation

Liquid marbles have promising applications in the field of microreactors, where the opening and closing of their surfaces plays a central role. We have levitated liquid water marbles using an acoustic levitator and, thereby, achieved the manipulation of the particle shell in a controlled manner. Upon increasing the sound intensity, the stable levitated liquid marble changes from a quasi-sphere to a flattened ellipsoid. Interestingly, a cavity on the particle shell can be produced on the polar areas, which can be completely healed when decreasing the sound intensity, allowing it to serve as a microreactor. The integral of the acoustic radiation pressure on the part of the particle surface protruding into air is responsible for particle migration from the center of the liquid marble to the edge. Our results demonstrate that the opening and closing of the liquid marble particle shell can be conveniently achieved via acoustic levitation, opening up a new possibility to manipulate liquid marbles coated with non-ferromagnetic particles.

Effect of particle hydrophobicity on the properties of liquid water marbles

We study both the static properties and dynamic behavior of liquid water marbles coated with silica nanoparticles of varied hydrophobicity. The static properties are characterized by the variation of marble height and diameter with increasing marble volume, such that the effective surface tension γeff of the marble can be obtained. The dynamic behavior of liquid marbles includes their impingement on a solid surface and their compression between two parallel glass plates. Marbles coated with particles of intermediate hydrophobicity exhibit maximum γeff values and enhanced mechanical robustness. Due to particle detachment from or particle rearrangement at the air–water interface caused by the impact, the dynamic surface tensions γd of liquid water marbles are different in magnitude to those of γeff. In fact, γd plays an important role in determining the contact time and oscillation period during the impact and rebound processes. Our results show that both the static effective and dynamic surface tension depend on the hydrophobicity of the particles coating the marble surfaces.

Tuning the wettability of an aluminum surface via a chemically deposited fractal dendrite structure

We have developed a straightforward method to tune the wettability of an aluminum substrate within a contact angle (CA) range from 2° to 170° by chemical deposition in CuCl2 solution and fluoroalkylsilane (FAS) modification. The CA of the as-deposited surface decreases with deposition time due to the growth of fractal copper dendrites, which enhance the surface roughness significantly. After subsequent modification with FAS, a superhydrophobic surface with CA 170° and sliding angle less than 5° has been obtained. With the increase of CA, the maximum spreading of water droplets is reduced. A bouncing behavior is observed for droplets impinging on the superhydrophobic substrate, suggesting its potential application as a self-cleaning surface.Graphical abstract

Liquid Marble Coalescence and Triggered Microreaction Driven by Acoustic Levitation

Liquid marbles show promising potential for application in the microreactor field. Control of the coalescence between two or among multiple liquid marbles is critical; however, the successful merging of two isolated marbles is difficult because of their mechanically robust particle shells. In this work, the coalescence of multiple liquid marbles was achieved via acoustic levitation. The dynamic behaviors of the liquid marbles were monitored by a high-speed camera. Driven by the sound field, the liquid marbles moved toward each other, collided, and eventually coalesced into a larger single marble. The underlying mechanisms of this process were probed via sound field simulation and acoustic radiation pressure calculation. The results indicated that the pressure gradient on the liquid marble surface favors the formation of a liquid bridge between the liquid marbles, resulting in their coalescence. A preliminary indicator reaction was induced by the coalescence of dual liquid marbles, which suggests that expected chemical reactions can be successfully triggered with multiple reagents contained in isolated liquid marbles via acoustic levitation.

Acoustic levitation of liquid drops: Dynamics, manipulation and phase transitions

The technique of acoustic levitation normally produces a standing wave and the potential well of the sound field can be used to trap small objects. Since no solid surface is involved it has been widely applied for the study of fluid physics, nucleation, bio/chemical processes, and various forms of soft matter. In this article, we survey the works on drop dynamics in acoustic levitation, focus on how the dynamic behavior is related to the rheological properties and discuss the possibility to develop a novel rheometer based on this technique. We review the methods and applications of acoustic levitation for the manipulation of both liquid and solid samples and emphasize the important progress made in the study of phase transitions and bio-chemical analysis. We also highlight the possible open areas for future research.

Rejuvenated bouncing of non-Newtonian droplet via nanoparticle enwrapping

We have studied the impact dynamics of non-Newtonian droplets containing poly-(ethylene oxide) (PEO) by using a high speed camera. To get a deeper insight into the droplet-substrate interaction on droplet impact behavior, both the bare droplet and the liquid marbles have been investigated. The usually observed anti-rebound phenomenon caused by the PEO additives can be rejuvenated by enwrapment of droplets with nanoparticles. The presence of PEO in bare droplets greatly enhances the contact line friction and leads to irreversible impalement of the surface structure. Whereas for liquid marbles, the nanoparticle shell on the droplet surface inhibits the impalement. Our results clearly demonstrate that it is the droplet-substrate friction rather than the bulk rheological properties of the liquid that plays the essential role in the anti-rebound effect.

Acoustic levitation of soap bubbles in air: Beyond the half-wavelength limit of sound

We report on the behavior of levitated soap bubbles in a single-axis acoustic field. For a single bubble, its surface in the polar regions is under compression, but in the equatorial region, it is under suction. Levitation becomes unstable when the height of the bubble approaches half the wavelength of the sound wave because horizontal fluctuations lead to a negative recovery force and a negative levitation force. Vertically stacked double bubbles notably can be stable under levitation if their total vertical length is ∼5λ/6, significantly beyond λ/2 in consequence of the formation of a toroidal high-pressure region around the waist of the two bubbles. Our results provide a deeper insight into the stability of acoustic levitation and the coupling between bubbles and sound field.

Sectorial oscillation of acoustically levitated nanoparticle-coated droplet

We have investigated the dynamics of a third mode sectorial oscillation of nanoparticle-coated droplets using acoustic levitation in combination with active modulation. The presence of nanoparticles at the droplet surface changes its oscillation amplitude and frequency. A model linking the interfacial rheology and oscillation dynamics has been proposed in which the compression modulus e of the particle layer is introduced into the analysis. The e obtained with the model is in good agreement with that obtained by the Wilhelmy plate approach, highlighting the important role of interfacial rheological properties in the sectorial oscillation of droplets.

Superhydrophobic polytetrafluoroethylene surfaces by spray coating on porous and continuous substrates

We study the preparation of superhydrophobic polytetrafluoroethylene (PTFE) coating by a spraying method with copper mesh and aluminum plate substrates. The hydrophobicity and toughness of the resulting surfaces are investigated using water contact angles, sliding angles, rubbing tests, and impact experiments. The heat treatment is found to be the key to adjusting the microstructure as well as the wettability. Sufficient heat accumulation at 230 °C through 50 min sintering gives rise to a hierarchical structure characterised by micro spheres with lots of nanopores on their surfaces, leading to excellent water repellence proved by the large contact angle, and small sliding angle, and the complete water drop rebounce. Insufficient heat accumulation with less than 20 min sintering at 230 °C results in micro spheres without nanopores; the resulting surface presents weak water repellence in all the property tests. In addition, the porous substrate of copper mesh is proved feasible for the spraying preparation of a tough superhydrophobic surface, although it is not as good as the continuous substrate of aluminum plates.