Ferrofluid droplet breakup process and neck evolution under steady and pulse-width modulated magnetic fields

• Published in: Journal of molecular liquids 2021-12, Vol.343, p.117536, Article 117536
• Main Authors: Bijarchi, Mohamad Ali, Favakeh, Amirhossein, Mohammadi, Kaivan, Akbari, Ali, Shafii, Mohammad Behshad
• Format: Article
• Language: English
• Subjects: Droplet formation; Ferrofluid; Magnetic field; Neck evolution; Self-similarity
• ISSN: 0167-7322; 1873-3166
• DOI: 10.1016/j.molliq.2021.117536

[Display omitted] •Neck evolution during ferrofluid droplet formation from a capillary is investigated.•Self-similar behavior for different magnetic inductions and angles is observed.•A new pattern of neck size reduction is reported under alternating magnetic field.•The trend of variation is divided into slow-going and fast-going necking regions.•Self-similar pinch-off behavior is reported for various applied magnetic frequencies. Numerous applications in engineering and biotechnology have attracted the attention of many researchers to the analysis of underlying physical phenomena during the droplet pinch-off. In this study, the neck evolution during the formation of a ferrofluid droplet from a capillary is investigated under two types of magnetic field for a drop-on-demand system. The two types are steady and Pulse-Width Modulated (PWM) magnetic fields. First, under steady magnetic field, the necking process is studied for different values of magnetic Bond number and various angles between magnetic coil centerline and gravity. Subsequently, self-similar behavior in the vicinity of the detachment moment is observed. In the last stage of droplet pinch-off, a scaling law is fitted to the variation of minimum neck diameter versus remaining time to the breakup. The scaling exponent is obtained to be 0.67 which is in good agreement with that for the ferrofluid droplet formation using a syringe pump reported in previous studies. Second, under PWM magnetic field, the necking process during the detachment of a ferrofluid droplet is explored and compared with that in the presence of the steady magnetic field. A new pattern of minimum neck diameter variation with remaining time to the breakup is observed for the first time. The trend of variation can be divided into two regions which are called “slow-going” and “fast-going” necking. Two mechanisms are introduced which explain the transition from the slow-going to fast-going necking region. The effect of magnetic Bond number and dimensionless applied frequency on the neck evolution is studied. In the proximity of the detachment moment, a universal behavior for pinch-off process is observed and the exponent of the fitted scaling law is equal to 0.84. Finally, the variation of relative neck position under the PWM magnetic field is compared with that in the presence of steady magnetic field.