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We experimentally investigate the evaporation kinetics of a sessile ferrofluid droplet placed on a soft substrate in the presence of a time-dependent magnetic field. We use both bright field visualization techniques and μ-PIV analysis to gain qualitative as well as quantitative insights into the internal hydrodynamics of the droplet. The results show that the droplet evaporation rate is augmented significantly in the presence of a time-dependent magnetic field, attributed primarily to the enhanced internal flow advection. We show that the motion of the magnetic nanoparticles dictates the overall life-time of the evaporating ferrofluid droplet. At lower frequencies of the magnetic field, the magnetic nanoparticles move towards the magnet and agglomerate into a chain-like cluster formation, oriented according to the magnetic field lines. On the other hand, at higher frequencies, the magnetic nanoparticles do not have sufficient time to travel the whole characteristic length (droplet diameter). Consequently, we observe the presence of a critical frequency at which the perturbation time scale balances the advective time scale of the flow inside the droplet. We show that on account for this balance between the time scales, the droplet experiences a minimum life-time. Finally, we demonstrate that the evaporation kinetics of a ferrofluid droplet in the presence of a time-dependent magnetic field can be described through three distinguishable stages viz., the decreasing contact angle and variable radius zone, the decreasing contact angle and decreasing radius zone and the late mixed zone. The inferences drawn from this study could have far-reaching implications in fields ranging from biomedical engineering to surface patterning.

Convective flow of single-phase ferrofluids under the influence of constant and alternating magnetic field has attracted attention as an effective strategy for enhanced heat transfer in mini/micro thermal systems. In the present study, an attempt has been made to gain deep insight of the heat transfer characteristics of single-phase ferrofluid flow in a heated stainless steel tube under the influence of constant and time-varying magnetic field. The governing parameters are mainly the magnetic flux density (B) and perturbation frequency (f) of the applied magnetic field. Three magnetic flux density value of B = 0 G, 700 G and 1080 G have been used for constant magnetic field. Constant value of B = 1080 G was used for alternating magnetic field while, frequencies of applied magnetic field has been varied from 0.1 Hz to 5 Hz. Flow Reynolds number was kept constant to Re = 66. Some 2-D numerical simulations have also been performed to qualitatively support the experimental data. The study is focused to delineate the mechanism of augmentation of heat transfer through the interaction of available force fields, i.e., interplay of magnetic force and inertia of the flow, and also the effect of various time scales on the flow and thermal behavior. Major inferences of the study are (a) on the application of external magnetic (constant and alternating), heat transfer augments (b) existence of a threshold frequency of external magnetic field for maximum augmentation as outcome of advective time-scale and magnetic perturbation time-scale. InfraRed Thermography (IRT) has been used to measure the wall temperature, while, some bright field visualizations have also been done to qualitatively support the explanations of experimental data.

We report the experimental investigations on the mixing of a ferrofluid droplet with a non-magnetic fluid in the presence of a time-dependent magnetic field on an open surface microfluidic platform. The bright field visualization technique, in combination with the micro-PIV analysis, is carried out to explore the internal hydrodynamics of the ferrofluid droplet. Also, using the Laser-induced fluorescence (micro-LIF) technique, we quantify the mass transfer occurring between the two droplets, which in effect, determines the underlying mixing performance under the modulation of the frequency of the applied magnetic field. We show that the magnetic nanoparticles exhibit complex spatio-temporal movement inside the ferrofluid droplet domain under the influence of a time-dependent magnetic field, which, in turn, promotes the mixing efficiency in the convective mixing regime. Our analysis establishes that the movement of magnetic nanoparticles in presence of the time-periodic field strengthens the interfacial instability, which acts like a sparking agent to initiate an augmented mixing in the present scenario. By performing numerical simulations, we also review the onset of interfacial instability, mainly stemming from the susceptibility mismatch between the magnetic and non-magnetic fluids. Inferences of the present analysis, which focuses on the simple, wireless, robust, and low-cost open surface micromixing mechanism, will provide a potential solution for rapid droplets mixing without requiring pH level or ion concentration dependency of the fluids., 34 pages, 14 figures

In the present investigation, a ferrofluid-based cooling method is suggested for the photovoltaic thermal (PVT) systems. Perturbing the ferrofluidic flow domain by an electromagnet can be an effective mechanism to alter the thermal flow characteristics. The temporal evolution of the ferrofluid flow domain under the influence of a constant and a time-varying magnetic field is observed by infrared thermography. The study has been conducted for three magnetic strength of \(\overline{B} = 0G\), \(\overline{B} = 700G\), and \(\overline{B} = 1080G\), while the magnetic field frequencies are varied from 0.1 to 5 Hz. The primary objective of the investigation is to outline the mechanism of enhancement in heat transfer by exploring the role played by the various force fields, i.e., the interplay between the magnetic and the inertia force field. Also, the intricate interplay between the involved timescale, such as advective, diffusive, and magnetic perturbation timescales and its subsequent role on the thermal characteristics of the flow field, is explored in detail. Major inferences of the study are (a) on the application of external magnetic (constant and alternating) and heat transfer augments (b), and there exists a critical frequency (of the perturbing electromagnet) at which the augmentation is maximum; this frequency is a resultant outcome of the balance between the advective timescale and magnetic perturbation timescale. The inferences drawn from this investigation will have far-reaching implications in the purview of designing an effective thermal management in the photovoltaic systems.

In this paper, a ferrofluid-based cooling technique is proposed for solar photovoltaic (PV) systems, where ferrofluid flow can be easily altered by the application of an external magnetic field leading to enhanced heat transfer from the hot surface of PV systems. The effect of both constant and alternating magnetic field on ferrofluid flow through a minichannel is explored numerically in the present work. A detailed parametric study is performed to investigate the effect of actuation frequencies of alternating magnetic field (0.5–20 Hz) and Reynolds numbers (Re = 24, 60, and 100) on heat transfer characteristics of ferrofluid. An overall enhancement of 17.41% is observed for heat transfer of ferrofluid in the presence of magnetic field compared to the base case of no magnetic field. For the case of alternating magnetic field, a critical actuation frequency is observed for each Reynolds number above which heat transfer is observed to decrease. The enhancement or decrease in heat transfer of ferrofluid is found to depend on several factors such as actuation frequency of alternating magnetic field, Reynolds numbers of ferrofluid flow, and formation/dispersion of stagnant layers of ferrofluid at the magnet location. Preliminary visualization of ferrofluid flow is also carried out to provide a qualitative insight to the nature of transportation of ferrofluid in the presence of an alternating magnetic field.

The present paper reports actuation of sessile ferrofluid droplets over a hydrophobic substrate in the presence of a time-dependent magnetic field generated by an electromagnet. The internal hydrodynamics of the ferrofluid droplet in the presence of magnetic field are measured using both bright field visualization and micro-particle image velocimetry (μ-PIV) techniques. During ON cycle of the magnetic field, bright field visualizations show the migration of nanoparticles towards the contact line near the vicinity of the electromagnet resulting in aggregation of nanoparticles inside the droplet. Similarly, aggregated nanoparticles at the contact line from the ON cycle are observed to disperse from the cluster of nanoparticles during the OFF cycle of the magnetic field. Both migration and dispersion of nanoparticles result in bulk motion inside the ferrofluid droplet during the ON and OFF cycle of the magnetic field. Velocity measurements from μ-PIV technique successfully validate the qualitative measurements of flow field from bright field visualization technique. A critical frequency is observed for the applied magnetic field above which negligible dispersion of nanoparticles resulted inside the ferrofluid droplet during the OFF cycle of the magnetic field.