Film thickness measurements in liquid–liquid slug flow regimes

Abstract

a b s t r a c tAt present there is significant interest in the development of small scale medical diagnostic equipment. These devices offer faster processing times and require smaller sample volumes than equivalent macro scale systems. Although significant attention has been focused upon their outputs, little attention has been devoted to the detailed fluid mechanics that govern the flow mechanisms within these devices. Conventionally, the samples in these small scale devices are segmented into distinct discrete droplets or slugs which are suspended in an organic carrier phase. Separating these slugs from the channel wall is a very thin film of the organic carrier phase.The magnitude of this film is the focus of the present study and the effects of sample slug length and carrier phase fluidic properties on the film are examined over a range of Capillary numbers. A non-intrusive optical technique was used to capture images of the flow from which the magnitude of the film was determined.The experimental results show that the film is not constant along the length of the slug; however above a threshold value for slug length, a region of constant film thickness exists. When compared with existing correlations in the literature, the experimental data showed reasonable agreement with the Bretherton model when the Capillary number was calculated based on the mean two phase flow velocity. However, significant differences were observed when the Capillary number was redefined to account for the mean velocity at the liquid interface, i.e., the mean slug velocity.Analysis of the experimental data revealed that it fell into two distinct flow regimes; a visco-capillary regime and a visco-inertial regime. A modified Taylor expression is presented to estimate the magnitude of the film for flows in the visco-capillary regime while a new model is put forward, based on Capillary and Weber numbers, for flows in the visco-inertial regime. Overall, this study provides some novel insights into parameters, such as aqueous slug length and carrier phase fluidic properties, that affect the thickness of the film in liquid-liquid slug flow regimes.

Local Nusselt number enhancements in liquid–liquid Taylor flows

“…6 was calculated using Eq. (5), which was developed by the current authors, [31], in a previous work examining the thickness of the liquid film in liquid-liquid flows: …”

“…The flows then merged inside either a segmenter or T-junction to create well-ordered streams of aqueous droplets separated from one another by oil slugs. Segmenters work by periodically creating and rupturing a liquid bridge between two opposing capillary tips and have been used by the authors in previous works examining the hydrodynamics of liquid-liquid slug flows [31,32]. Segmenters require the reservoir to be filled with a density matched oil to ensure that a buoyancy free environment exists for axis-symmetric liquid bridging.…”

Review and extension of pressure drop models applied to Taylor flow regimes

“…While in the liquid-liquid analysis, T-junctions were used in conjunction with segmenters. Segmenters work by periodically creating and rupturing a liquid bridge between two opposing capillary tips and have been used previously by Curran et al (2005) and Mac Giolla Eain et al (2013) in liquid-liquid slug flow studies. Segmenters require a reservoir to be filled with a density matched oil to ensure that a buoyancy free environment exists for axisymmetric liquid bridging.…”

“…However, it has been shown by a number of authors, (Aussillous and Quere, 2000;Grimes et al, 2007;Mac Giolla Eain et al, 2013), that this assumption is invalid and that the theoretical solution of Bretherton breaks down at Ca < 10 À4 and Ca > 10…”

Modeling of Slug Velocity and Pressure Drop in Gas‐Liquid‐Liquid Slug Flow

“…Mac Giolla Eain et al [12] modified the film thickness calculation for liquid-liquid systems by fitting an empirical model based on the Bretherton equation to their experimental data for the viscocapillary regime with Ca < 0.12 (Eq. (5)).…”