Effect of Wettability on the Collision Behavior of Acoustically Excited Droplets

Acoustic droplet ejection (ADE) is a noncontact technique for micro-liquid handling (usually nanoliters or picoliters) that is not restricted by nozzles and enables high-throughput liquid dispensing without sacrificing precision. It is widely regarded as the most advanced solution for liquid handling in large-scale drug screening. Stable coalescence of the acoustically excited droplets on the target substrate is a fundamental requirement during the application of the ADE system. However, it is challenging to investigate the collision behavior of nanoliter droplets flying upward during the ADE. In particular, the dependence of the droplet’s collision behavior on substrate wettability and droplet velocity has yet to be thoroughly analyzed. In this paper, the kinetic processes of binary droplet collisions were investigated experimentally for different wettability substrate surfaces. Four states occur as the droplet collision velocity increases: coalescence after minor deformation, complete rebound, coalescence during rebound, and direct coalescence. For the hydrophilic substrate, there are wider ranges of Weber number (We) and Reynolds number (Re) in the complete rebound state. And with the decrease of the substrate wettability, the critical Weber and Reynolds numbers for the coalescence during rebound and the direct coalescence decrease. It is further revealed that the hydrophilic substrate is susceptible to droplet rebound because the sessile droplet has a larger radius of curvature and the viscous energy dissipation is greater. Besides, the prediction model of the maximum spreading diameter was established by modifying the droplet morphology in the complete rebound state. It is found that, under the same Weber and Reynolds numbers, droplet collisions on the hydrophilic substrate achieve a smaller maximum spreading coefficient and greater viscous energy dissipation, so the hydrophilic substrate is prone to droplet bounce

Microzone Melting Method of Porous Reactor Fabrication with Structure-Controlled Microchannel Networks for High Yield In Situ DNA Synthesis

This paper presents a simple and cost-effective method for fabricating porous polydimethylsiloxane (PDMS) reactor array chip that is applied in de novo DNA synthesis. A microzone melting technique is proposed in the preparation of a porous PDMS reactor using the sugar particle as a sacrificial template. The curing temperature of 155 °C, higher than the melting point of the sugar particle, is chosen to enhance interconnectivity and reduce internal surface roughness of micropores inside the porous PDMS. The morphological observation and flow resistance test were performed on porous PDMS fabricated with various sugar particle sizes and weight ratios of PDMS to the sugar particle. The results indicate that region I (interconnected pore area) plays a pivotal role in the flow resistance of the porous PDMS reactor. The effectiveness of the porous PDMS reactor in DNA synthesis is verified by gel electrophoresis and fluorescence hybridization. Synthesis product analysis demonstrates that the yield of the porous PDMS reactor is in the same order of magnitude as that of a commercially available 200 nmol synthesis column. The proposed porous PDMS microreactor array chip exhibits great potential in the high-yield DNA synthesis

Sticker Microfluidics: A Method for Fabrication of Customized Monolithic Microfluidics

This paper proposes a novel strategy and an all-in-one toolbox that allows instrument-free customization of integrated microfluidic systems. Unlike the modular design of combining multiple microfluidic chips in the previous literature, this work, for the first time, proposes a “template sticker” method, in which sacrificial templates for microfluidic components are batch-produced in the form of standardized stickers and packaged into a toolbox. To create a customized monolithic microfluidic system, the end users only need to select and combine various template stickers following formulated steps. The fabricated microfluidic devices have well-defined microscale features, while the fabrication process is inexpensive and time-saving. Various functional microfluidic devices were fabricated and tested using this toolbox. The capability to create microchannels on curved surfaces is also demonstrated. As a proof of concept, we developed with the proposed toolbox a colorimetric testing platform for the detection of nitrite ions. The sticker toolbox, as the first self-contained portable platform for microfluidic fabrication, allows prompt customization of monolithic devices, enabling deployment of microfluidics with both ideal performance and customizability