Developments on Wetting Effects in Microfluidic Slug Flow

Abstract

Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tensions forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived realistic, and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel Tjunction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed for insight 2 into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channels walls and highlight the value of numeral simulation in microfluidic design.

Effect of mixing on mass transfer characterization in continuous slugs and dispersed droplets in biphasic slug flow microreactors

“…For example, with the same resazurin system, Yang et al [33] reported that the measured k L a was reduced along the channel length as τ = 1-5 s. This is because: (1) the large channel dimension (w = h = 2 mm) abated the intensity of internal circulation; (2) the reaction rate of oxygen with low resazurin concentration (0.1 g/L, corresponding C max = 0.199 × 10 -3 mol/L vs. C*=0.255 × 10 -3 mol/ L) might be smaller than its physical transport rate, especially when resazurin conversion approached 100%, leading to the presence of oxygen molecules at the interface or even in the slug bulk. In a recent work of the same group [34], the time-related k L a was also presented in an FEP circular tube (d = 1 mm, 0.1 g/L resazurin solution) under nonwetting slug flow (i.e., the liquid film was absent), which largely attenuated the internal mixing [51]. In comparison, the wetting slug flow within rectangular microchannel in the current work induced evident leakage flow and intensive internal circulation (will be explained in the following text), together with the low conversion of resazurin (mostly < 60%) allowed us to obtain the intrinsic k L a in the current residence time range.…”

Pore-Scale Analysis of Condensate Blockage Mitigation by Wettability Alteration

“…It was identified that, at a low Reynolds number (Re ≈ 60) airflow, drag force on water droplets is minimal, allowing droplets to grow until the channel is obstructed and subsequently be convected by airflow. Santos and Kawaji [43] investigated numerically wetting effects in microfluidic slug flow. They verified that the significant velocity slip developed in hydrophilic microchannels [44][45][46] is absent when the channel walls are hydrophobic, since gas slugs fill the channel cross section.…”

Hydrodynamics and mechanism of hydrophobic foam column tray: Contact angle hysteresis effect

“…Many emerging studies have emphasized the interfacial wettability effects on multiphase transport processes. Specifically, interfacial wettability effects have been investigated in terms of fluid dynamics (drag reduction and wet particle collision), hydrodynamics (membrane bubbling and sieve tray), heat transfer and anti‐icing, oil recovery, oil–water separation, micro‐reactor technology, and so forth.…”