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The research on the application of nanofluids in the thermal management of electric components is gaining widespread attention. In this context, and as past studies on nanofluids mainly focus on thermal properties, the investigation of the electrical properties of nanofluids has become important. The role of dispersants on the electrical conductivity of nanofluids stabilized using two biopolymer dispersants, chitosan, and gum arabica (GA), and a synthetic dispersant Sodium Dodecyl Benzoic Sulfate (SDBS) is investigated in this work. The nanofluids are prepared with water, nanoparticles (0.1 wt%), and dispersants (0.05 and 0.5 wt%), and the electrical conductivity is measured to examine the role of nanoparticle materials (CuO, Fe3O4, and CuO + Fe3O4), dispersants, and temperatures (25–40 °C). According to observations, the dispersants significantly affect the electrical conductivity, while nanoparticle material has a minimal impact. At 0.5 wt% concentration, SDBS stabilized nanofluids have electrical conductivity 708.7 times higher than water, whereas, with chitosan and GA, it is 144.2 and 12.8 times higher than water, respectively. CuO-water nanofluids show relatively higher electrical conductivity than Fe3O4 and hybrid CuO + Fe3O4 water nanofluids. Measured electrical conductivity is used for regression analysis using response surface methodology (RSM). Three quadratic nonlinear polynomial equations are developed to predict the electrical conductivity based on the input parameters (dispersant concentration and temperature). The predicted and measured values were in excellent agreement with each other. The current study offers a novel perspective on the electrical behavior of nanofluids with biopolymer dispersants and provides a prediction model for nanofluid electrical conductivity.

The optimization of process parameters of the nanofluid preparation process for maximum stability and high heat transfer is an active and important area of research. In this work, the effect of the surfactant material, surfactant weight, and ultrasonication time are studied on distilled water-based CuO, Fe3O4, and CuO+Fe3O4 nanofluids. Taguchi L9 orthogonal array was used for the design of the experiment and 9 samples were prepared using this array. The effect of each level of process parameter on the thermal conductivity is analyzed by calculating Signal to Noise Ratio (SNR) and optimum levels of these parameters are identified. The crucial role of stability in delivering high thermal conductivity nanofluids as predicted by SNR analysis is further confirmed using Analysis of Variance (ANOVA).

Microfluidic methods act as an effective motile sperm separation technique used in infertility treatments. This work presents a standalone microfluidic device to separate motile sperm cells from non-motile sperm cells and debris. The separation mechanism is based on the centrifugal force acting on sperms and the ability of progressive motile sperms to swim upstream. The separation of motile sperm is carried out using a simple T-shaped microchannel which constitutes three reservoirs: one inlet and two outlets. Herein, one of the outlets is kept sealed. The sealed channel leads to a high-velocity gradient and a rheotaxis zone at the T junction resulting in the separation of motile sperms. Separated sperms are isolated in a sealed channel with a low Reynolds number flow so that sperms cannot have a net displacement, which ensures that the sperms do not re-enter the fluid flow. CFD simulation is conducted to study the flow fields inside the channel and experimental investigation is carried to observe the separation behaviour of sperms. The reported device provides 100% sperm separation efficiency and ensures the entrapment of sperm cells for a longer period. A modified colorimetric nitroblue tetrazolium test conducted on separated sperm cells shows that there is only a marginal increase in superoxide (O(2)(−)) production, proving normal sperm integrity. This device offers an effective and safe alternative to conventional sperm sorting methods. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13534-022-00229-9.

White blood cells (WBCs) reside close enough to the endothelium vascular wall to detect a variety of chemical signals and combat bacterial and viral diseases in the human body. It is vital to understand the phenomenon of WBC margination since it is an essential mechanism in microcirculation which aids fighting infections. Several factors influence WBC margination, including hematocrit (Hct), flow rate, red blood cell (RBC) aggregation, RBC and WBC deformability, and the width of RBC free layer. WBC dynamics is strongly influenced by the presence of RBCs. In this study, we investigate WBC margination by varying the size and shapes of RBCs. The change in size and shape of RBCs is achieved by altering the tonicity of the blood sample. The margination phenomenon is studied at different values of hematocrits (3%–40% Hct) and flow rates (0.2–1 μl min−1). The different values of hematocrits is achieved by diluting the whole human blood using normal saline (0.9% NaCl), hypotonic saline (0.45% NaCl), and hypertonic saline (3% NaCl) solutions, respectively. Experiments are conducted using three different geometrical microchannels; straight, curved, and constriction-expansion (CE). The findings of hypotonic and hypertonic saline solutions are compared to the results of normal saline solutions. It is found that hypotonic and hypertonic solutions have minimum effect on WBC margination in a curved channel; however, in the case of straight and CE channel margination improves. When blood cells are diluted with hypotonic saline, WBC margination is shown to be highest in CE microchannels, whereas for straight microchannel, the hypertonic solution provides the best margination. We also report particle dynamics within the microchannel and compare their behavior with experimental results for Hct 3%. This study provides critical information on WBC margination in situations where RBCs deviate from their normal shape and size.

In this study, the effect of surfactant on the stability of hybrid nanofluids is explored. The thermal, rheological, and thermo-rheological performance of the three most stable nanofluid samples (one each of CuO, Fe3O4 and CuO + Fe3O4) is studied in detail. Stability analysis is carried out by monitoring the agglomerations in the nanofluids over a period of 20 days. The behaviour of stability indicators (i.e., zeta potential and hydrodynamic diameter) shows that the nanofluid stability is highly dependent on the type of surfactant utilized. It is seen that CuO-water nanofluid shows a maximum 5.47 % enhancement in thermal conductivity compared to its base fluid, while Fe3O4-water and hybrid CuO + Fe3O4-water nanofluids show a maximum enhancement of 3.11 % and 3.95 %, respectively. The results also show that the presence of CTAB surfactant increases the viscosity of the nanofluid. The contact angles for all the nanofluids are lower than that of the base fluid, confirming their superior wettability characteristics. The thermal performance of the nanofluids is also assessed by determining the property enhancement ratio and the figure of merit.

The arguments in favor of using nanofluids in thermal applications have been increasing substantially for the last few decades. Nanofluids provide improved performance in heat transfer processes in comparison to their base fluids as a result of their superior thermal properties. Even though nanofluids exhibit better thermal properties, their usage has been limited due to their stability issues. The stability of the nanofluid greatly affects its thermal properties over a period of time. The stability and thermal properties of nanofluids can be affected by parameters like surfactants used and their concentrations, and also on the nanomaterial used in the nanofluid. In this study, surfactant material and nanoparticle material are selected as process variables and for each variable two levels are selected. For surfactant material, Sodium Lauryl Sulfate (SLS) and Cetyl Trimethyl Ammonium Bromide (CTAB) are selected. Surfactant concentration ratios are taken as 1:2 for CuO and 1:4 for Fe3O4 material. Four nanofluid samples are prepared with 0.1% weight of nanoparticles and their stability and properties are studied. The feasibility of turbidity as an indicator for stability is also explored in this work. The results show that the zeta potential and hydrodynamic characteristics are largely dependent on the surfactant material. Both surfactants show good stability of nanofluids. In-line with earlier observations, it is also observed that the nanoparticle material has a dominant effect on the thermal conductivity of the nanofluids. Comparing the turbidity of the nanofluids to the zeta potential, it is observed that the turbidity measurement gives first-hand information about the stability of nanofluids and can act as an index for stability. But still, more exploration is necessary for this field so a quantitive relation can be established between turbidity and zeta potential of different nanofluid materials and concentrations.

This paper presents an experimental study of heat transfer in a slightly rarefied gas flowing in a circular tube with constant wall temperature boundary condition. Local temperature measurements are carried out for the first time in rarefied gas flows to investigate into the anomalous values of Nusselt number (Nu) obtained by a previous experimental study (Demsis et al., 2009). The present measurements agree well with the low Nu reported by Demsis et al. (2009) when the Nu is obtained using their procedure; additionally, the measurements reveal the importance of end effects in determining the Nusselt number in rarefied gases. The Nusselt number obtained in the present experiments tends to zero with increasing axial conduction. Nu shows a weak dependence on Knudsen number for the range investigated here (0.001 < Kn < 0.012).

This paper presents a comparative study of the flow of liquid and gases in microchannels of converging and diverging cross sections. Towards this, the static pressure across the microchannels is measured for different flow rates of the two fluids. The study includes both experimental and numerical investigations, thus providing several useful insights into the local information of flow parameters as well. Three different microchannels of varying angles of convergence/divergence (4 degrees, 8 degrees and 12 degrees) are studied to understand the effect of the angle on flow properties such as pressure drop, Poiseuille number and diodicity. A comparison of the forces involved in liquid and gas flows shows their relative significance and effect on the flow structure. A diodic effect corresponds to a difference in the flow resistance in a microchannel of varying cross section, when the flow is subjected alternatively to converging and diverging orientations. In the present experiments, the diodic effect is observed for both liquid and gas as working fluids. The effect of governing parameters-Reynolds number and Knudsen number, on the diodicity is analysed. Based on these results, a comparison of design perspectives that may be useful in the design of converging/diverging microchannels for liquid and gas flows is provided.

This work presents the study of isothermal rarefied gas flows in converging microchannels. Experiments are carried out on microchannels of three different converging angles (4 degrees , 8 degrees , and 12 degrees). Numerical investigation is carried out using commercial software to study the local behaviour of the flow parameters. The simulations show a sudden drop in the fluid temperature at the exit of the microchannel. Knudsen minimum, which was experimentally observed for the first time recently in diverging microchannels, is also noted here in the case of flow in converging cross section. It is interesting to note that, at the location of Knudsen minimum, the Knudsen number and the value of the minimum mass flow rate are same for both converging and diverging cross sections, for all the angles tested. This result implies the absence of any flow preference at high Knudsen numbers when the flow is subjected to converging and diverging orientations of the microchannel. Published by AIP Publishing.

With the aim of exploring potential applications of Pulsating Heat Pipes (PHP), for space/terrestrial sectors, experimental study of embedded PHP thermal radiators, having two different effective Biot numbers respectively, and subjected to conjugate heat transfer conditions on their surface, i.e., natural convection and radiation, has been carried out under different thermo-mechanical boundary conditions. High resolution infrared camera is used to obtain spatial temperature profiles of the radiators. To complement the experimental study, detailed 3D computational heat transfer simulation has also been undertaken. By embedding PHP structures, it was possible to make the net thermal resistance of the mild steel radiator plate equivalent to the aluminum radiator plate, in spite of the large difference in their respective thermal conductivities ( k Al ∼ 4 k MS ). The study reveals that embedded PHP structures can be beneficial only under certain boundary conditions. The degree of isothermalization achieved in these structures strongly depends on its effective Biot number. The relative advantage of embedded PHP is appreciably higher if the thermal conductivity of the radiator plate material itself is low. The study indicates that the effective thermal conductivity of embedded PHP structure is of the order of 400 W/mK to 2300 W/mK, depending on the operating conditions.

Study of rarefied gas flow in converging and diverging cross sections is crucial to the development of micro-nozzles and micro-thrusters. In other practical cases too, a microchannel may not always be straight and may include diverging and converging sections in the flow path. In this context, isothermal rarefied gas flow in microchannels of longitudinally varying cross section is studied experimentally in this work. The primary objective is to investigate the existence of Knudsen minimum in microchannels of varying cross sections. The effect of geometrical cross section and fluid properties on the Knudsen minimum are also investigated by performing experiments on three divergence angles (4 degrees, 8 degrees, and 12 degrees) and three different gases (argon, nitrogen, and oxygen) to prove the robustness of the result. The Knudsen minimum, which is one of the characteristic features of rarefied flows, is experimentally observed for the first time in a microchannel of varying cross section. The position of the Knudsen minimum (at Kn approximate to 1) is seen to depend only weakly on the divergence angle and fluid properties. (C) 2016 AIP Publishing LLC.

Rarefied gas flow is gaining increasing importance with the emergence of Micro Electro Mechanical Systems (MEMS). Knudsen minima is one of the characteristic feature of such rarefied flows and has been observed in uniform cross section channels such as plane channel, cylindrical tube and annulus. However, data pertaining to gaseous flow in varying cross section channel is relatively sparse. Channels of varying cross section are frequently encountered in MEMS devices and are fundamental to the design of micro-scale nozzles and micro-valves. In this context, rarefied gas flow through a diverging microchannel (divergence angle – 12 degree) is studied experimentally with three different gases (argon, nitrogen and oxygen). The experiments are performed over a wide range with the mean Knudsen number varying from slip to the transitional regime (0.07 to 1.2). It is found that the effect of molecular weight of the gas on the non-dimensional mass flow rate is negligible. The Knudsen minima is experimentally observed for the first time in microchannel of non-uniform cross section.