Tapas Kumar Mandal

An optical probe for liquid–liquid two-phase flows

A non-intrusive optical probe has been developed for the identification of flow patterns during liquid–liquid two-phase flow through a conduit. It is based on the difference in optical properties of the respective phases and works on the basis of the proportion of light attenuated and scattered by the two-phase mixture. The measuring system consists of a laser source, a light-dependent resistance (LDR) and a processing circuit. The source and the detector (LDR) are located on opposite sides of the test pipe to detect light after its passage through the test section containing the two-phase mixture. The LDR generates a variable resistance depending on the intensity of light incident on it. The voltage across the resistance is amplified by a three-stage amplifier circuit. The dc output of the circuit is recorded as time series signal and analysed for flow patterns during liquid–liquid flow through horizontal and vertical tubes. The probe provides similar signals for similar phase distributions in both vertical and horizontal pipes and could be applied successfully to differentiate between dispersed and separated flows in the two tube orientations. It has also been observed that the present method of detection is more effective as compared to a parallel wire conductivity probe in identifying flow patterns at high phase velocities.

Prediction of flow pattern of gas-liquid flow through circular microchannel using probabilistic neural network

The present study attempts to develop a flow pattern indicator for gas-liquid flow in microchannel with the help of artificial neural network (ANN). Out of many neural networks present in literature, probabilistic neural network (PNN) has been chosen for the present study due to its speed in operation and accuracy in pattern recognition. The inbuilt code in MATLAB R2008a has been used to develop the PNN. During training, superficial velocity of gas and liquid phase, channel diameter, angle of inclination and fluid properties such as density, viscosity and surface tension have been considered as the governing parameters of the flow pattern. Data has been collected from the literature for air-water and nitrogen-water flow through different circular microchannel diameters (0.53, 0.25, 0.100 and 0.050mm for nitrogen-water and 0.53, 0.22mm for air-water). For the convenience of the study, the flow patterns available in literature have been classified into six categories namely; bubbly, slug, annular, churn, liquid ring and liquid lump flow. Single PNN model is unable to predict the flow pattern for the whole range (0.53mm-0.050mm) of microchannel diameter. That is why two separate PNN models has been developed to predict the flow patterns of gas-liquid flow through different channel diameter, one for diameter ranging from 0.53mm to 0.22mm and another for 0.100mm-0.05mm. The predicted map and their transition boundaries have been compared with the corresponding experimental data and have been found to be in good agreement. Whereas accuracy in prediction of transition boundary obtained from available analytical models used for conventional channel is less for all diameter of channel as compared to the present work. The percentage accuracy of PNN (~94% for 0.53mm ID and ~73% for 0.100mm ID channel) has also been found to be higher than the model based on Weber number (~86% for 0.53mm ID and ~36% for 0.05mm ID channel).

Localized electric field induced transition and miniaturization of two-phase flow patterns inside microchannels

Strategic application of external electrostatic field on a pressure‐driven two‐phase flow inside a microchannel can transform the stratified or slug flow patterns into droplets. The localized electrohydrodynamic stress at the interface of the immiscible liquids can engender a liquid‐dielectrophoretic deformation, which disrupts the balance of the viscous, capillary, and inertial forces of a pressure‐driven flow to engender such flow morphologies. Interestingly, the size, shape, and frequency of the droplets can be tuned by varying the field intensity, location of the electric field, surface properties of the channel or fluids, viscosity ratio of the fluids, and the flow ratio of the phases. Higher field intensity with lower interfacial tension is found to facilitate the oil droplet formation with a higher throughput inside the hydrophilic microchannels. The method is successful in breaking down the regular pressure‐driven flow patterns even when the fluid inlets are exchanged in the microchannel. The simulations identify the conditions to develop interesting flow morphologies, such as (i) an array of miniaturized spherical or hemispherical or elongated oil drops in continuous water phase, (ii) “oil‐in‐water” microemulsion with varying size and shape of oil droplets. The results reported can be of significance in improving the efficiency of multiphase microreactors where the flow patterns composed of droplets are preferred because of the availability of higher interfacial area for reactions or heat and mass exchange.

Prediction of rise velocity of a liquid Taylor bubble in a vertical tube

In the present study, a theoretical analysis has been performed to predict rise velocity of liquid Taylor bubbles. The viscous potential flow analysis has been adopted for this purpose. The results have revealed the importance of nose shape. This has been quantified by a shape factor. A method for estimating this factor has also been proposed.

Magnetically guided chemical locomotion of self-propelling paperbots

Self-propelling microjets with multimodal chemical and magnetic controls for motion were prepared from printed waste papers coated with MnO2 nanoparticles. Because the magnetic remote control was infused from the ferromagnetic coating of the printer ink, the nanoparticles decomposed peroxide fuel to induce locomotion by ejecting oxygen bubbles through a microjet. The paper microjets could be loaded with fluorescent rhodamine 6G (R6G), a model payload, before being remotely guided through an external magnetic field inside a fluidic environment. The reported methodology provides an economic, scalable, and biodegradable scheme for the design and development of bubble-propelled microengines, which are capable of directed locomotion.

The flow of magnetic nanoparticles in magnetic drug targeting

Magnetic drug targeting has been explored by an in vitro study of the deposition of polyvinyl alcohol (PVA) coated magnetic carrier nanoparticles (MCNPs) in a tube under the influence of an externally applied magnetic field. Experiments and simulations show a steady decrease in the retention of MCNPs with increasing flow rate and weaker magnetic field strength. The retention of MCNPs has been significantly influenced by the fluid flow behaviour resulting from the position and shape of the magnet, magnetic properties and size of the MCNPs, and the magnetic field strength. Under strong magnetic fields, the MCNPs tend to creep along the wall of the tube and undergo high shear before reaching the targeted region. These results highlight the importance of choosing the region of MCNP injection, magnetic field strength and, the magnetic properties and size of the MCNPs to minimize the loss of the drug.

Digitization of two-phase flow patterns in a microchannel induced by an external AC field

An externally applied alternating current (AC) electrostatic field can deform the interface of a pair of weakly conducting liquids to engender droplet flow patterns inside the ‘T’ shaped microchannels. The electrohydrodynamic stresses originating from the accumulation of free and induced charges at the interface of the immiscible liquids stimulate the formation of droplets with higher surface to volume ratio. Strikingly, the size, shape, and frequency of the flow patterns can be tuned by varying the frequency and waveform of the external AC field. The enhanced dielectrophoretic force at a higher field intensity and lower frequency of the AC field facilitates the formation of droplets with smaller size and higher throughput. The size, shape, and frequency of the droplets are also found to be functions of the ratio of the electrical conductivity of the phases and the interfacial tension. The proposed methodology demonstrates a non-invasive pathway to digitize the flow patterns inside a multiphase microfluidic device with the help of an external AC field.

Liquid Taylor Bubbles Rising in a Vertical Column of a Heavier Liquid: An Approximate Analysis

It has been noted that a volume of lighter liquid when injected into a stationary column of a heavier liquid, it rises up as a simple elongated Taylor bubble. In the present study, experimental and theoretical analyses have been performed to understand the rise of liquid Taylor bubbles. The experiments have been performed with different liquid pairs with their viscosities ranging from 0.71 mPa s to 1.75 mPa s and conduit sizes ranging from 0.012 m to 0.0461 m. The bubble shape has been predicted using a potential flow analysis and validated from photographic measurements. This analysis has been further modified to predict the rise velocity. The modified analysis accounts for the density difference between the two liquids, viscosity effects of the primary liquid, and interfacial tension of two fluids. A semi-empirical equation has been developed, which gives satisfactory results for most of the cases.

Discrete electric field mediated droplet splitting in microchannels: Fission, Cascade, and Rayleigh modes

Numerical simulations supplemented by experiments together uncovered that strategic integration of discrete electric fields in a non‐invasive manner could substantially miniaturize the droplets into smaller parts in a pressure driven oil‐water flow inside microchannels. The Maxwells stress generated from the electric field at the oil–water interface could deform, stretch, neck, pin, and disintegrate a droplet into many miniaturized daughter droplets, which eventually ushered a one‐step method to form water‐in‐oil microemulsion employing microchannels. The interplay between electrostatic, inertial, capillary, and viscous forces led to various pathways of droplet breaking, namely, fission, cascade, or Rayleigh modes. While a localized electric field in the fission mode could split a droplet into a number of daughter droplets of smaller size, the cascade or the Rayleigh mode led to the formation of an array of miniaturized droplets when multiple electrodes generating different field intensities were ingeniously assembled around the microchannel. The droplets size and frequency could be tuned by varying the field intensity, channel diameter, electrode locations, interfacial tension, and flow ratio. The proposed methodology shows a simple methodology to transform a microdroplet into an array of miniaturized ones inside a straight microchannel for enhanced mass, energy, and momentum transfer, and higher throughput.

Electric field mediated spraying of miniaturized droplets inside microchannel

We report a facile and noninvasive way to disintegrate a microdroplet into a string of further miniaturized ones under the influence of an external electrohydrodynamic field inside a microchannel. The deformation and breakup of the droplet was engendered by the Maxwells stress originating from the accumulation of induced and free charges at the oil–water interface. While at smaller field intensities, for example less than 1 MV/m, the droplet deformed into a plug, at relatively higher field intensities, e.g. ∼1.16 MV/m, a pair of droplets having opposite surface charge was formed. The charged droplets showed an interesting periodic bridging and breakup during their translation motion across the channel. For even higher field intensities, for example more than 1.2 MV/m, the entire droplet underwent dielectrophoresis toward one of the electrodes before experiencing a strong attractive force from the other electrode to deform into a shape of a Taylor cone. With progress in time, mimicking the electrospraying phenomenon, the cone tip periodically ejected a string of miniaturized water droplets to form a microemulsion inside the channel. The frequency and size of the droplet ejection could be tuned by varying the applied field intensity. A water droplet of ∼214 μm diameter could continuously eject droplets of size ∼10 μm or even smaller to form a microemulsion inside the channel.