arXiv subjects

S. K. Saroj

Publications and source records attributed to S. K. Saroj.

3 recordsLinked to original sources

arXiv2026

Evaporative cooling and deposition patterns of evaporating $Al_2O_3$ nanofluid droplets

The present study examines evaporative cooling and the resulting deposition patterns of a sessile $Al_2O_3$-based nanofluid droplet on a hydrophobic glass substrate at different temperatures. Evaporation predominantly occurs in the pinned contact line mode for both heated and non-heated cases, with only slight recession observed without heating. The droplet height and contact angle decrease linearly with time, and scaling relations are proposed to describe the evolution of droplet geometry and volume. A non-dimensional parameter, $Π_{rel}$, is introduced to characterize transitions in deposition patterns. For $Π_{rel} \leq 1$ ($T_s \leq 26^\circ$C), interconnected irregular polygonal network structures form at the periphery, which are rarely reported in evaporating droplets. With increasing substrate temperature, this structure is suppressed, giving rise to a classical coffee-ring pattern for $1 < Π_{rel} \leq 10$. At higher temperatures ($T_s > 40^\circ$C), dual-ring formation along with central particle deposition is observed for $Π_{rel} > 10$. The interfacial temperature is higher near the contact line and decreases toward the apex, and a universal scaling for the temperature profile is proposed. Internal flow velocity increases with substrate temperature, exhibiting asymmetric multi-vortex structures. Evaporative cooling intensifies with heating, enhancing evaporation flux and capillary flow. Appropriate scaling relations for evaporation flux and capillary velocity are established. Overall, the dynamics are governed by thermocapillary (Marangoni) flow induced by evaporative cooling, which enhances internal circulation and governs nanoparticle deposition morphology.

physics.flu-dyn↗

arXiv2026

Frequency-Dependent Magnetic modulation of deposition morphology

This paper presents a novel approach for magnetic modulation of deposition morphology in an evaporating ferrofluid droplet. The magnetic field strength and ferrofluid concentration are kept unchanged, while the actuation frequencies are varied from 0.016 Hz to 5 Hz. In the absence of a magnetic field, a coffee-ring formation is observed and consistent with previous studies\cite{deegan1997capillary,deegan2000contact,saroj2019drying}. The application of a time-dependent magnetic field significantly modifies the deposition morphology. The periodic magnetic field induces the formation of multiple concentric rings during evaporation. The number of rings initially increases with increasing actuation frequency of the electromagnet. However, beyond a critical actuation frequency ($f_c = 0.2\,\text{Hz}$), the number of rings decreases. At higher actuation frequencies, magnetic particles preferentially deposit in the central region of the droplet, resulting in suppression of the coffee-ring effect. Additionally, the thickness of the inner rings and the ring spacing decrease with increasing actuation frequency up to critical actuation frequency. The transition from multi-ring formation to coffee-ring suppression is governed by the competition among magnetic forcing, capillary flow, and particle diffusion. The underlying physical mechanisms responsible for droplet dynamics and deposition morphology under periodic magnetic fields are evaluated using scaling arguments. The results demonstrate that diffusive particle transport plays a dominant role in determining the deposition pattern. A non-dimensional magnetic switching number, based on the magnetic perturbation timescale, is introduced as a control parameter to characterize the frequency-dependent deposition behavior.

physics.flu-dyn↗

arXiv2024

Charged drop impinging on particles dispersed over a metallic plate: A method of particle cleaning

An electric field applied to a droplet impinging on a hydrophobic surface has an extensive variety of applications, including ant-icing, heat transfer enhancement, self-cleaning, droplet manipulation, and electrostatic spraying. The present study demonstrates an effective method of particle removal using a charged droplet. This method employs a pin-plate electrode setup to investigate the dynamics of a charged droplet impact on the surface covered with particles. The particles of different properties such as wettability, electrical conductivity, etc. have been used. Silane-coated glass beads, carbon black, and glass beads are dispersed over the ground copper electrode. The applied potential is also varied from 2 kV to 4 kV. A high-speed imaging is employed to visualize the drop motion, dynamic behavior, and self-cleaning phenomenon. The experimental results indicate that drop generation and impact occur at applied potentials of 2.5, 3, and 3.5 kV, in contrast, at 2 kV, there is no droplet pinch-off. At 4 kV, electric breakdown and bridging of the droplet between the capillary and ground electrode are observed. The drop impact on the silane-coated glass bead leads to their attachment due to the adhesiveness of the particles and the droplet. The silane-coated particles are removed from the droplet surface due to the deformation of the drop and the electric repulsive force. In the case of carbon black and glass beads, the particles are captured by the droplet due to the electrostatic force of attraction. Higher electric potentials lead to an increased spreading diameter of the droplet. The higher electric field enhances the contact area between the droplet and the particles, thereby removing more particles.

physics.flu-dyn↗