Department of Mechanical engineering
Permanent URI for this collectionhttp://localhost:4000/handle/123456789/1921
Browse
3 results
Search Results
Item Confined Evaporation-Mediated Enhanced Residence Time of Levitated Water Drops over Deep Oil Pools(ACS, 2021-11) Harikrishnan, A.R.We observe the impact of bouncing and floating of water drops on a pool of immiscible volatile oil pools at low Weber numbers. The residence time of the impacting drop ranges from a few milliseconds to a few seconds before it sinks into the lighter oil phase. It is hypothesized that the confined evaporation from the volatile oil pool replenishes the thin film draining and results in prolonged floating and delayed sinking of drops into the oil pool. Water drops are released from a low height to impact on volatile hydrocarbon oil deep pools of various volatilities. The floating dynamics and residence times are captured using high-speed imaging. A theoretical model for the residence time has been developed to evaluate the hypothesis. The drop residence time is found to be directly proportional to the volatility of the oil pool in accordance with the hypothesis. The mathematical model incorporating the coupled confined evaporation and film draining dynamics is found to be in well agreement with the experimentally observed residence time. The bouncing–sinking regime map has been developed based on the experimental data. Supporting InformationItem Competitive Electrohydrodynamic and Electrosolutal Advection Arrests Evaporation Kinetics of Droplets(ACS, 2020-07) Harikrishnan, A.R.This article reports the hitherto unreported phenomenon of arrested evaporation dynamics in pendant droplets because of electric field stimulus. The evaporation kinetics of pendant droplets of electrically conducting saline solutions in the presence of a transverse, alternating electric field is investigated experimentally. While the increase of field strength reduces the evaporation rate, increment in field frequency has the opposite effect. The same has been explained on the solvation kinetics of ions in polar water. Theoretical analysis reveals that change in surface tension and the diffusion-driven evaporation model cannot predict the decelerated evaporation. With the aid of particle image velocimetry, suppression of internal circulation velocity within the droplet is observed under electric field stimulus, which directly affects the evaporation rate. A mathematical scaling model is proposed to quantify the effects of electrohydrodynamic circulation and electrothermal and electrosolutal advection on the evaporation kinetics. The analysis encompasses major governing parameters, namely, the thermal and solutal Marangoni numbers, the electrohydrodynamic number, the electro-Prandtl and electro-Schmidt numbers, and their respective contributions. It has been shown that the electrothermal Marangoni effect is suppressed by the electric field, leading to deteriorated evaporation rates. Additionally, the electrosolutal Marangoni effect further suppresses the internal advection, further reducing the evaporation rate by a larger proportion. Stability analysis reveals that the electric body force retards the stable internal advection. The stability mapping also illustrates that if the field strength is high enough for the electrosolutal advection to overshadow the solutal Marangoni effect completely, it can lead to improvement in evaporation rates.Item Optical thermogeneration induced enhanced evaporation kinetics in pendant nanofluid droplets(Elsevier, 2018-03) Harikrishnan, A.R.Optofluidic manipulation using lasers and colloidal systems have immense applications in microscale thermofluidic devices. The present study analyses the effect of laser as an external optical source in modulating the evaporation characteristics of pendent nanofluid microdroplets (which are free from substrate effects) so as to capture the physics behind the interfacial mass transport. The study analyses the effect of the power of laser, nature and concentration of the particle on evaporation rate of the complex fluids. Evidence of internal circulation was observed with Particle Image Velocimetry (PIV) technique in colloidal systems which together with the volumetric heat generation due to laser irradiation can be attributed to be the cause behind the enhanced evaporation rate. The nanoparticles are observed to efficiently convert the energy of radiant photons to heat while water is observed to show poor evaporative performance under laser irradiation. Theoretical analysis of the evaporation rate with the classical diffusion driven evaporation model is found to fall short in predicting the evaporation rate in colloidal systems, even in the absence of laser irradiation. Thermal Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found not potent enough to induce the circulation in such systems which could improve evaporation. Hence the observed weak internal circulation can be attributed to the solutal Marangoni arising out of the local concentration gradients of nanoparticles in the droplet and the enhanced Thermophoretic drift and Brownian dynamics of the nanoparticles. Also the enhancement in the diffusion coefficient and its strong dependence with the particle concentration is also contributing to the enhancement in enhanced evaporation rate in nanocolloidal systems. The augmentation in the evaporative process under laser radiation is found to be governed majorly by the optical heat generation with weak solutal Marangoni and a scaling model is able to accurately predict the experimental observations. The present findings could have implications in modulation of thermofluidic and species transport phenomena in microscale devices.