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We analytically investigated the electroosmotic flow characteristics of complex viscoelastic liquids within a charged hydrophobic microchannel, considering the pH and salt concentration-dependent surface charge effects in our analysis. We examined the variation of the electric-double layer (EDL) potential field, the surface charge-dependent slip (SCDS) length, the flow field, the viscosity ratio, and both normal and shear stresses in relation to the bulk pH, bulk salt concentration, and Deborah number of the solution. Our current findings indicate that, under strong flow resistance due to increased electrical attraction on counter ions, a highly basic solution with a high EDL potential magnitude results in a significant decrease in the slip length. Neglecting the effect of SCDS leads to an overestimation of flow velocity, with this overprediction being more pronounced for highly basic solutions. This overestimation diminishes as bulk salt concentration increases, particularly when compared to strongly acidic solutions. Furthermore, a noticeable increase in average velocity is observed as the Deborah number rises for highly basic solutions compared to highly acidic ones. This is attributed to the substantial reduction in apparent viscosity caused by the shear-thinning nature of the liquid at higher shear rates, supported by a larger zeta potential modulated strong electrical force for basic solutions. Additionally, we found that the intensity of shear and normal stresses tends to increase with bulk pH, primarily due to the rise in electric body force at higher zeta potential. These results can potentially inform the design and development of a compact, nonmoving electroosmotic pump for transporting biological species with varying physiological properties, such as solution pH. This technology could be applied in subsequent processes involving mixing, separation, flow-focusing for cell sorting, and other related applications. [ABSTRACT FROM AUTHOR]

At the microfluidic scale, the utilization of helically coiled channels (HCCs), also known as a spiral channel, for two-phase flow offers numerous advantages in various applications. Existing articles mainly focus on the macro-scale transport, examining secondary flows induced in curved channels. The increasing demand, however, for innovative miniature equipment for thermal energy management emphasizes the importance of comprehending gas–liquid micro-scale flow in curved channels. Unfortunately, despite a vast body of literature on this paradigm, there is still a lack of systematic investigations into the underlying facets of two-phase micro-scale transport in HCCs. To address this gap, our study conducted experiments on adiabatic two-phase air–water flow inside an up-flow helical micro-scale tube. The tube had a hydraulic diameter of 0.87 mm, a coil diameter of 50 mm, and a helical pitch of 20 mm. The primary aim was to explore the impact of centrifugal force on flow pattern, void fraction, and frictional pressure drop characteristics. Additionally, we carefully examined the phase separation phenomenon influenced by the secondary flows induced by the curved channel. In particular, we compared the gas-core flow pattern (either throat-annular flow or annular flow), void fraction, and frictional pressure drop obtained from our experiments on the helical tube with corresponding results based on straight micro-scale channel configurations for an Eötvös number of approximately 0.01. In summary, this study delves deep into the crucial aspects of two-phase micro-scale transport in HCCs, contributing to a better understanding of these systems for future advancements in micro-channel applications. [ABSTRACT FROM AUTHOR]

With a focus on biochemical applications and utilizing relevant physical properties, the current study numerically analyzes the impact of electroosmotic vortex and fluid rheology on the chemical reaction characteristics of species. This is achieved by installing integrated positively charged patches on the extended region of the microreactor with three inlets for injecting the reactants and generating the electroosmotic vortex. In order to produce species "C" in the extended region of the microreactor, it is presumed that reactant species "A" is injected through the upper and lower inlets and reactant species "B" is injected via the intermediate inlet. To solve the associated transport equations with appropriate boundary conditions, a thorough theoretical framework is developed. The results show that the ability of the reactant species to react is boosted when vortices form in the microreactor, increasing the convective mixing strength for reactant species. Furthermore, the fluid rheology significantly affects the reaction characteristics, which is a noteworthy finding. For fluids exhibiting a higher shear-thinning nature, the average concentration of the produced species follows an increasing–decreasing trend with the Carreau number. Additionally, it becomes apparent that the influence of the Damkohler number on the average generated species concentration is negligible at lower Carreau numbers, but it increases with the Damkohler number at higher Carreau numbers. The study also reveals that both rheological and chemical parameters have a substantial impact on the flow rate of product species. Overall, the findings of this investigation provide valuable insights for the development of technologically advanced electroosmotic microreactor capable of effectively generating the intended product species. [ABSTRACT FROM AUTHOR]

The gravitational force effect is of concern when designing flow boiling systems deployed on Earth and in space. A study of the R-134a flow boiling inside the horizontal and vertical channels was experimentally performed to explore the flow orientations' effect on flow pattern and heat transfer characteristics. The test section was a 0.5 mm diameter tube from which the corner effect was excluded. The single channel was used to avoid disturbances that flow maldistribution could possibly cause in the multiple channels. The experimental two-phase flow corresponded to a 1.75 confinement number, which was above the threshold for micro-scale flow. The results indicated the flow regime map, which was less affected by the channel orientation except for that slug flow pattern that was unable to survive during the vertical downflow and vertical upflow. Also, based on the present micro-scale flow, the heat transfer results were mostly independent of the gravitational force effect, and the nucleate boiling and convective mechanisms tended to govern them. [ABSTRACT FROM AUTHOR]

We make an effort in this study to experimentally investigate the flow boiling pressure drop characteristics in a multi-microchannel heat sink. For the multi-microchannel heat sink, 27 parallel microfluidic channels are considered. Microchannels are fabricated on the copper block and have a hydraulic diameter of 421 µm and length of 40 mm. We perform experiments considering the refrigerant, considered the working fluid in this study. A high mass flux (G ∼ 400 kg/m2 s–1200 kg/m2 s) for a range of wall heat flux (q′′ ∼ 10 kW/m2–170 kW/m2) and varying degrees of saturation temperature (Tsat ∼ 13 °C, 18 °C, and 23 °C) are considered for the experiments. We demonstrate the correlative effect of the total pressure drop on the two-phase flow pattern in the channel. By depicting the interplay between the total pressure drop (∆p) and the frictional pressure gradient, we show that the frictional pressure gradient increases with the increasing mass flux while it decreases with the increasing degree of saturation temperature. We show the significant impact of the heat flux and saturation temperature on the frictional pressure gradient. In addition, we develop a new correlation for the pressure drop characteristics taking the interference effect of the frictional pressure drop in the multi-microchannel heat sink. The inferences of this experimental study will have far-ranging consequences for the design of heat exchangers, leading to the optimization of microscale thermal management equipment. [ABSTRACT FROM AUTHOR]

We report the experimental investigations of two-phase flow boiling heat transfer characteristics of a refrigerant in a microfluidic channel at a high mass flux (more than 1000 kg/m2 s). We investigate the heat transfer coefficients at a heat flux range of 7.63 kW/m2–49.46 kW/m2, mass flux range of 600 kg/m2 s–1400 kg/m2 s (high mass flux), and saturation temperature range of 23 °C–31 °C. We propose the new two-phase flow boiling heat transfer correlation of a refrigerant, which is used as the working fluid for the present experiments, at the microfluidic scale. We experimentally establish the functional relationship of two-phase flow boiling heat transfer correlation of the refrigerant during flow boiling in a rectangular microchannel with the Reynolds number, the boiling number, and the Weber number. We believe that the inferences of this study may provide a design basis for the micro-heat exchanger, typically used for thermal management in electronic devices, micro-electro-mechanical systems, and electric vehicle battery cooling system. [ABSTRACT FROM AUTHOR]

We investigate the flow dynamics of an incompressible fluid in a polyelectrolyte grafted rotating narrow fluidic channel under the influence of an externally applied electric field. Here, we invoke an analytical formalism to solve the transport equations governing the flow dynamics in the rotating environment. We bring out the rotational force driven complex flow dynamics in the channel as modulated by the soft layer induced alteration in the electrostatic potential under electrokinetic actuation. We observe that the flow reverses at the centre of the channel for higher rotational speeds, emerging from an intricate competition among the rotation induced Coriolis force and the electrical body force due to the electrical double layer phenomenon. We show that an increase in the thickness of the polyelectrolyte layer (PEL) increases the transverse electrostatic potential, which upon interacting with the externally applied electric field alters the flow dynamics non-trivially in a rotating platform. Furthermore, we show that the frictional drag, stemming from the presence of ions in polymeric chains in the PEL enhances the resistance to the flow field, leading to a reduction in flow velocities in the channel. Finally, we explain the consequential effects of grafted PEL as realized through the thickness of the layer and the PEL drag on the alteration in the volume transport rates in the channel. [ABSTRACT FROM AUTHOR]