Uniform generation of water slugs in air flowing through superhydrophobic microchannels with T-junction

Abstract

This article describes an approach to render all walls of a rectangular microchannel superhydrophobic by coating them with hydrophobically modified silica nanoparticles. Differences between the coated nanoparticles in terms of feature size and structure resulted in different contact angle hysteresis (CAH) on the channel walls. The generation of water slugs in a continuous air flow was experimentally studied in the superhydrophobic microchannels with T-junction. The breakup mechanism of water slugs was proved to accord with the squeezing regime by analyzing the data collected from pressure sensor and high-speed camera at the T-junction. A home-made optical measurement system was employed to investigate the uniformity of the water slugs, which was characterized by the standard deviation and coefficient of variation of slug length. The results showed that superhydrophobic microchannels with CAH lower than 15° could produce uniform water slugs in a slug flow pattern, whereas those with CAH of nearly 30° led to unstable flow. Lower CAH and higher gas flow rate were preferred to reduce slug length and improve slug uniformity. An empirical scaling law was established to predict the slug length and was based on the finding that the total length and number of water slugs in the microchannel were controlled by the volumetric flow ratio of water to air and water flow rate, respectively. This work provides a novel microfluidic platform to generate uniform liquid-in-gas slugs under moderate ranges of air flow rate from 5 mL min−1 to 30 mL min−1 (6 < ReG < 40) and volumetric flow ratio of water to air from 1/300 to 1/2, and this performance benefits from superhydrophobic coatings with low CAH.