Formation and breakup of compound pendant drops at the tip of a capillary and its effect on upstream velocity fluctuations

Abstract

In this paper, the formation and breakup process of compound pendant drops (CPDs, pendant drops with smaller drops or bubbles in them) at the tip of a glass capillary and its effect on upstream velocity fluctuation are experimentally investigated. The formation process of an air/water compound drop from a CPD consists of four main stages. First, an air plug in the capillary flows into the small liquid pendant drop to initialize a small CPD. Next, a liquid slug flows into the CPD, and the liquid in the CPD accumulates. Subsequently, an air plug flows into the CPD, and it coalesces with the existing air bubble in the CPD. The accumulation and coalescence stages repeat, until the CPD reaches a critical weight, then the CPD finally breaks up to produce a compound drop. For the air/SDS-solution system, the bubbles in the CPDs do not coalesce, and the contact line of the CPDs initially climbs along the capillary and then moves downwards with the growth of the CPDs. The upstream velocity fluctuates during the periodical formation and breakup of the CPD due to Laplace pressure variation at the tip of the glass capillary. By adding surfactant into water, the fluctuation of the upstream velocity decreases. The size distribution of the compound drops produced by the breakup of CPDs is quantified, and the results show that the current system is able to produce monodisperse compound drops.

Flow structure of compound droplets moving in microchannels

“…In contrast, with the development of micro-fabrication techniques during the past two decades, microfluidics offers an alternate route to produce monodisperse compound droplets one by one, and the droplet properties can be tuned precisely 2 . In microfluidics, compound droplets can be produced by forming the inner and the outer droplets subsequently at two droplet formation units [4][5][6][7][8][9] (such as two T-junctions 4,5 or two flow-focusing geometries 6,7 ) or by forming the inner and the outer droplets simultaneously by properly combining two droplet formation units (such as a microcapillary structure proposed by Utada et al 10,11 ). To understand the formation of compound droplets, several numerical studies have been carried out to simulate the formation process in different microchannel structures [12][13][14] , and the effects of relevant parameters have been reported, such as the geometry of the microfluidic device and the flow rates of different phases.…”

A new strategy for highly efficient single-drop microextraction with a liquid–gas compound pendant drop

“…13 In recent years, compound pendant drop (CPD, a liquid drop containing a smaller droplet or bubble inside) microextraction has received signicant attention due to its many promising applications. [14][15][16] The research on CPD microextraction can be traced to the introduction of SDME in 1996, when Dasgupta et al developed a drop-in-drop system for a ow injection extraction. 17 However, the development of liquid-gas CPD microextraction was stagnant because the formation of smaller bubbles in a micro-droplet had long been thought of as a disadvantage in SDME.…”

Uniformity of gas and liquid two phases flowing through two microchannels in parallel

“…Procedure of image processing to obtain lengths in each photograph: (a) importing one image to MATLAB using the imread.m file; (b) focusing on a region of one microchannel, and subtracting the background; (c) converting to the black-and-white image[24]; (d) digitalizing to 0 or 1 for distinguishing gas or liquid.…”