Ferrofluid, also known as magnetic fluid, is a nano-scale functional material, usually prepared by co-precipitation method[1]. It has both the magnetic properties of a solid and the fluidity of a liquid. When there is no external magnetic field, the magnetic liquid is a Newtonian fluid. When a magnetic field is applied, the magnetic liquid exhibits magnetism and exhibits a non-Newtonian fluid behavior[2].
Ferrofluid is a colloidal suspension of single-domain magnetic particles, which is composed of three parts, namely the magnetic particles, the surfactant and the base carrier liquid[3,4]. The magnetic particles are in a diameter size ranging from 8–10 nm, coated with surfactant to keep liquid from agglomerating, dispersed in a base liquid. In the presence of a magnetic field, the magnetic particles will be reorganized making the whole body of fluid strongly magnetized.
Since first synthesis of a ferrofluid was produced by Papell[5] in 1965, it has been applied in numerous industry fields[6]: dynamic sealing[7], heat conductor, dampers[8], and drug targeting[9], etc. Among them, the magnetic fluid seal is a new type of sealing method, which is different from the traditional sealing method in the past. The magnetic fluid seal has the advantages of zero leakage, long life, and high reliability[10]. In the practical application of the magnetic fluid sealing device, the magnetic fluid exists in the gap between the pole tooth and the rotating shaft, forming several "O"-shaped sealing rings, which achieves the sealing effect[11,12]. However, under the action of a magnetic field and a temperature field, the magnetic fluid will evaporate, causing changes in the properties of the magnetic fluid, resulting in a seal failure. To satisfy high quality standard and harsh conditions in practical applications, like monocrystalline silicon furnace and optical device, it is necessary to have further research on evaporation of ferrofluid.
To date, some studies have been done from different aspects. Bottenberg et al.[13] prepared a polyphenylene ether-based ferrofluid. Compared with hydrocarbon-based ferrofluids, this ferrofluid has a lower saturated vapor pressure, which can reach 10-7torr at 20°C. It is especially suitable for Sealing under high vacuum environment. Kanno et al.[14] prepared a PFPE based ferrofluid, which had a vapor pressure of 7.0ⅹ10–10 Pa at 293 K and a resistance to active gasses. A PFPE based ferrofluid was prepared and characterized by Black et al.[15], which was found to have low volatility under high temperature.Li et al.[16] calculated the evaporation rate of hydrocarbon based and fluorocarbon based ferrofluid under four different vacuum levels. Cristaldo et al.[17–19] numerically analyzed heating process of ferrofluid droplet under an alternating magnetic field with a thermal boundary layer model. Bolotov et al. derived equations of evaporation rate to estimate the life of a tribounit with ferrofluid in the ambience of a vacuum[20] and a gas[21]. Jaiswal et al.[22] and Chattopadhyay et al.[23] studied the evaporation kinetics of a magnetic salt solution pendent droplet under a horizontal magnetic field. Experimental observations revealed that the evaporation rate enhanced with a magnetic field and magneto-solutal advection was thought to be the domineering factor in augmented evaporation rate. Mudra Jadav et al.[24] placed water-based magnetic droplets on a flat glass substrate and studied the influence of a magnetic field on the evaporation rate and contact angle. The results showed that in the dry droplets, the structure and distribution of the nanomagnetic particles are related to the direction and magnitude of the applied field. Saedi, M et al.[25] used a two-phase flow model and volume control technology to numerically study the boiling flow of pure fluid and nanofluid in a spiral tube. Studies have shown that the diameter of the coil affects the heat transfer coefficient, and it is found that the viscosity and density of the nanofluid also have an effect on the heat transfer coefficient. Mohammadpourfard, M et al.[26] simulated the nuclear pool boiling heat transfer of a ferrofluid on a horizontal plate. The effects of the negative and positive gradients of the magnetic field on the heat transfer rate and bubble shape are compared. Shyam Sudip et al.[27] studied the evaporation kinetics of a fixed ferromagnetic droplet placed on a soft substrate under the action of a time-dependent magnetic field. The time-varying magnetic field can effectively control the evaporation time of the ferromagnetic droplets, and a critical frequency of the applied magnetic field strength is determined, which makes the droplets encounter the minimum lifetime. At the critical frequency, the advection time scale of magnetic nanoparticles is balanced by the magnetic disturbance time scale.
Previous studies mainly focus on the characterization of ultra-low vapor pressure ferrofluid in a vacuum and theoretical analysis of evaporation process. However, there are lacks of researches to verify the equations being proposed and compare the difference of ferrofluid evaporation with and without a magnetic field in an experimental way. This paper analyzes the evaporation rate of a kerosene based ferrofluid in normal pressure with and without a magnetic field through experimental and theoretical studies.