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1. Modifying Wicking Speeds in Paper-Based Microfluidic Devices by Laser-Etching

   https://doi.org/10.3390/mi11080773  

   Kalish, Brent; Tan, Mick Kyle; Tsutsui, Hideaki (August 2020, Micromachines)

   Paper-based microfluidic devices are an attractive platform for developing low-cost, point-of-care diagnostic tools. As paper-based devices’ detection chemistries become more complex, more complicated devices are required, often entailing the sequential delivery of different liquids or reagents to reaction zones. Most research into flow control has been focused on introducing delays. However, delaying the flow can be problematic due to increased evaporation leading to sample loss. We report the use of a CO2 laser to uniformly etch the surface of the paper to modify wicking speeds in paper-based microfluidic devices. This technique can produce both wicking speed increases of up to 1.1× faster and decreases of up to 0.9× slower. Wicking speeds can be further enhanced by etching both sides of the paper, resulting in wicking 1.3× faster than unetched channels. Channels with lengthwise laser-etched grooves were also compared to uniformly etched channels, with the most heavily grooved channels wicking 1.9× faster than the fastest double-sided etched channels. Furthermore, sealing both sides of the channel in packing tape results in the most heavily etched channels, single-sided, double-sided, and grooved, wicking over 13× faster than unetched channels. By selectively etching individual channels, different combinations of sequential fluid delivery can be obtained without altering any channel geometry. Laser etching is a simple process that can be integrated into the patterning of the device and requires no additional materials or chemicals, enabling greater flow control for paper-based microfluidic devices. 

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2. Microchannel measurements of viscosity for both gases and liquids

   https://doi.org/10.1039/D1LC00202C  

   Shiba, Kota; Li, Guangming; Virot, Emmanuel; Yoshikawa, Genki; Weitz, David A. (July 2021, Lab on a Chip)

   We introduce a facile, microfluidic approach to mechanically measuring the viscosity of a fluid with high precision over a wide range, even extending from gases to liquids. 

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3. Hysteresis in spreading and retraction of liquid droplets on parallel fiber rails

   https://doi.org/10.1039/D1SM00126D  

   Wang, Fang; Schiller, Ulf D. (January 2021, Soft Matter)

   null (Ed.)

   Wetting and spreading of liquids on fibers occurs in many natural and artificial processes. Unlike on a planar substrate, a droplet attached to one or more fibers can assume several different shapes depending on geometrical parameters such as liquid volume and fiber size and distance. This paper presents lattice Boltzmann simulations of the morphology of liquid droplets on two parallel cylindrical fibers. We investigate the final shapes resulting from spreading of an initially spherical droplet deposited on the fibers and from retraction of an initial liquid column deposited between the fibers. We observe three possible equilibrium configurations: barrel-shaped droplet, droplet bridges, and liquid columns. We determine the complete morphology diagram for varying inter-fiber spacing and liquid volume and find a region of bistability that spans both the column regime and the droplet regime. We further present a simulation protocol that allows to probe the hysteresis of transitions between different shapes. The results provide insights into energies and forces associated with shape transformations of droplets on fibers that can be used to develop fiber-based materials and microfluidic systems for manipulation of liquids at small scale. 

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4. A Surfactant-Free Microfluidic Process For Fabrication Of Multi-Hollow Polyimide Aerogel Particles

   Yao, Y.; Joo, P.; Jana, S.C. (October 2020, International polymer processing)

   This work focuses on fabrication of multi-hollow polyimide gel and aerogel particles from a surfactant-free oil-in-oil emulsion system using a microfluidic droplet generator operating under dripping mode. The multi-hollow gel and aerogel particles have strong potential in thermal insulation. Under jetting and tip-streaming regime of microfluidic flows, droplets are generated with no occluded liquid phase. The present study investigates a means of designing polyimide gel particles with plurality of internal liquid droplets by strategically manipulating the flow rates of the continuous and dispersed phase liquids through the microfluidic droplet generator. The multi-hollow polyimide aerogel particles obtained after supercritical drying of the gel particles present mesopores, high BET surface area, and excellent prospect for thermal insulation. 

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5. Ignition of non-equilibrium methane dielectric barrier discharges in a multiphase plasma–liquid microfluidic device

   https://doi.org/10.1039/D5LC00090D  

   Das, Sudip; Meyer, Mackenzie; Kushner, Mark J; Hartman, Ryan L (April 2025, Lab on a Chip)

   Combined analysis of optical emission spectroscopy and infrared thermography revealing how liquid properties affect plasma ignition in a dielectric barrier discharge microfluidic system where methane-containing gas interacts with organic liquids. 

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