Experimental Investigation on the Dynamics of On-Demand Ferrofluid Drop Formation under a Pulse-Width-Modulated Nonuniform Magnetic Field

Abstract

Drop formation has been the focus of many studies because of its vast application in biomedicine and engineering, as well as its rich underlying physics. Applying a magnetic force on ferrofluids can provide more control over the formation process of the droplet. In this study, a time-dependent, nonuniform magnetic field was used for the formation of ferrofluid droplets using a nozzle. A pulse-width-modulation signal (PWM) was utilized to induce the time-dependent magnetic field, and a drop-on-demand system was designed using the capability of the PWM magnetic field. Three kinds of drop formation regimes under the PWM magnetic field were seen. Also, a new droplet generation regime was observed in which the drop is formed while it bounces back to the nozzle during the off-time period of the magnetic excitation. As compared to other techniques, the main advantage of droplet formation in this regime is that there will be no satellite droplet during the pinch-off. The regime map of drop formation based on the magnetic Bond number and the dimensionless induced frequency was obtained. Also, the effect of the duty cycle, the induced frequency, the magnetic induction, and the vertical interval between the coil's top surface and the nozzle on the drop formation evolution, the equivalent diameter of the droplets, the frequency of droplet formation, and the pulses that are necessary to form a drop was studied. Additionally, it was illustrated that by prolonging the duty cycle, the magnetic induction, or by decreasing the induced frequency, the equivalent diameter of the drop and the pulses that are necessary to form a drop reduce, while the frequency of drop formation increases. Eventually, a correlation for predicting the nondimensionalized diameter of the droplet, based on dimensionless variables, was presented with a maximum relative error of 8.1% and an average relative error equal to 2.2%.

On-demand ferrofluid droplet formation with non-linear magnetic permeability in the presence of high non-uniform magnetic fields

“…The values of the fixed dimensionless parameters and the range of variations of the contact angle and the magnetic Bond number are listed in the column determined by case A in Table 1 . As reported in the literature 49 , 50 , the surface tension force increases with increasing the wetted diameter of the nozzle; therefore, by increasing contact angle (nozzle hydrophobicity), the surface tension force decreases. By reducing this force, which resists the formation of the droplet, the droplet becomes smaller and forms faster.…”

On-demand Ferrofluid Droplet Formation With Non-linear Magnetic Permeability in the Presence of a Non-uniform Magnetic Field

“…The values of the fixed dimensionless parameters and the range of variations of the contact angle and the magnetic Bond number are listed in the column determined by case A in Table 1. As reported in the literature [45,46], the surface tension force increases with increasing the wetted diameter of the nozzle; therefore, by increasing contact angle (nozzle hydrophobicity), the surface tension force decreases. By reducing this force, which resists the formation of the droplet, the droplet becomes smaller and forms faster.…”

Magnetofluidic-based controlled droplet breakup: effect of non-uniform force field

“…on application of a magnetic field, its magnetization is comparable to any ferromagnetic particles, whereas, on removal of the applied field, the ferrofluid flow field does not display any net hysteresis (Rosensweig 1984). Ferrofluids have been successfully used in many engineering applications such as separation (Hejazian, Li & Nguyen 2015), heat transfer augmentation (Shyam et al 2019), mixing (Zhu & Nguyen 2012;Kitenbergs et al 2015), droplet generation (Tan et al 2010), breakup (Bijarchi & Shafii 2020;Bijarchi et al 2021) and many more (Rinaldi et al 2005;Cunha et al 2020). Although breakup of droplet in a T-junction has been widely explored, studies pertaining to ferrofluid droplet breakup under the modulation of a magnetic field are sparse.…”

“…2015), droplet generation (Tan et al. 2010), breakup (Bijarchi & Shafii 2020; Bijarchi et al. 2021) and many more (Rinaldi et al.…”