Macrodrop‐Impact‐Mediated Fluid Microdispensing

Abstract

High‐resolution fluid dispensing techniques play a critical role in modern digital microfluidics, micro‐biosensing, and advanced fabrication. Though most of existing dispensers can achieve precise and high‐throughput fluid dispensing, they suffer from some inherent problems, such as specially fabricated dispensing micronozzles/microtips, large operating systems, low volume tunability, and poor performance for low surface tension liquids and liquids containing solid/liquid additives. Herein, the authors propose a facile, low‐frequency micro dispensing technique based on the Rayleigh–Plateau instability of singular liquid jets, which are stimulated by the air cavity collapse arising in the impact of microliter drops on non‐wetting surfaces. This novel dispensing strategy is capable to produce single microdrops of low‐viscosity liquids with a tunable volume from picoliters to nanoliters, and the operational surface tension range covers most laboratory solvents. The dispensing function is implemented without using small‐dimension nozzles/tips and enables handling diverse complex liquids. Moreover, the rather simple operating platform allows the integration of the whole dispensing function into a handy portable device with a low cost. Employing this microdispensing technique, the authors have controlled microchemical reactions, handled liquid samples in biological analysis, and fabricated smart materials and devices. The authors envision that this rational microdrop generator would find applications in various research areas.

Successive Rebounds of Impinging Water Droplets on Superhydrophobic Surfaces

“…In Figure a, the restitution coefficients of all complete rebounds that we investigated are plotted as a function of the Weber number and the impact velocity. The scatter of the data shown in the figure is an inevitable phenomenon of experimental investigations of multiphase free-surface flows, including the impact of liquid droplets on solid surfaces, − where the underlying hydrodynamics are very complex. , The scatter of the restitution coefficient at low Weber numbers is also enhanced by the existence of inevitable surface defects due to the dominant role of the droplet–surface friction in energy dissipation, though much care has been taken in the surface fabrication. Indeed, existing studies of bouncing droplets on hot surfaces above the Leidenfrost temperature ,, and on sublimation solid surfaces such as dry ice, , where the impinging droplets are always suspended on a thin gas layer without contact with the solid surfaces, reported weakly scattered experimental data of diverse impact characteristics.…”

“…4 We point out that a high-speed jet can be observed during the recoiling of an impinging low-viscosity droplet on solid surfaces when the impact velocity is sufficiently high. 33,38,52 This jet is produced by the collapse of the air cavity, which is formed via the generation and propagation of the capillary wave upon impact. 33,53 In the experiments, such a jet flow was observed at 0.39 m/s ≲ V I ≲ 0.64 m/s, and the jet velocity increases from 1.9 to 10.4 m/s as jet radius R j decreases from 50 to 20 μm.…”

Overflow Control for Sustainable Development by Superwetting Surface with Biomimetic Structure

“…Although tremendous progress has been made to manipulate multiscale overflow on various solid–liquid interfaces, the practical industrial process involves more complex working conditions. It imposes stricter requirements on the controllability of overflow on different interfaces, such as the “solid–liquid–liquid”, “solid–air–liquid” system, and dynamic wetting with phase transition. ,, In addition, selective overflow on the organism’s surface is realized by a surface energy gradient and structural gradient. The overflow control technique needs to cooperate with micro-nanostructures, surface wettabilities, and external fields to realize the more diverse overflow manipulation and adapt to the constantly changing environmental conditions.…”

Superhydrophobic Strategy for Nature-Inspired Rotating Microfliers: Enhancing Spreading, Reducing Contact Time, and Weakening Impact Force of Raindrops

“…The impact of droplets on solid surfaces is a common hydrodynamic phenomenon that contains complex hydrodynamic mechanisms. − Upon the impacting, the droplet may exhibit diverse outcomes, including deposition, splashing, bouncing, crushing, etc., − depending on the droplet parameters (i.e., diameter, velocity, and shape), − fluid properties (i.e., viscosity, surface tension, and density), − surface wettability, − and micro/macrostructures. − In recent decades, learning from the lotus effect, − many superhydrophobic surfaces (water contact angle exceeds 150°) have been fabricated and expected to be applied in various fields. − Owing to its high repellency to water, the superhydrophobic surface shows prospective advantages for the protection of microfliers in rainy weather.…”