Experimental and Numerical Investigation of Ferrofluid Droplet Formation under Magnetic Field with Biotechnology Application

Bijarchi, Mohammad Ali | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53337 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Shafii, Mohammad Behshad
  7. Abstract:
  8. Droplet-based microfluidics offers a promising tool in engineering and biomedicine such as drug delivery, biochemistry, sample handling in lab-on-a-chip devices, and tissue engineering. These systems provide the droplets manipulation that can be used as reactors without any contamination for medical purposes. Utilizing magnetic force and ferrofluid droplets can help to better control over the droplet formation and manipulation. Considering the importance of this issue and its rich underlying physics, in this dissertation, ferrofluid droplets formation under a time-dependent, non-uniform magnetic field is studied. A Pulse-Width-Modulation signal (PWM) is utilized to apply the time-dependent magnetic field, and a Drop-on-Demand system is proposed. The ferrofluid droplet formation is investigated in the three different geometries, including nozzle, T-junction, and digital magnetofluidics. Satellite droplet formation has been reported as an undesirable phenomenon. Using the proposed idea, new regimes called “bounce-back” and “swinging” have been observed in droplet formation from the nozzle without any satellite droplet generation. The effect of magnetic flux density, duty cycle, applied magnetic frequency, the vertical distance between the coil’s top surface and the nozzle, and the angle of the magnetic coil with respect to gravity, along with corresponding dimensionless numbers on the drop formation evolution, the equivalent diameter of the droplets, formation frequency, and the pulses that are necessary to form a droplet is studied. Results show that by increasing magnetic flux density, and duty cycle, and decreasing the applied magnetic frequency, the droplet diameter decreases, whereas the formation frequency increases. Also, the minimum droplet diameter is observed at the distance of 10 mm and the angle of 45 degrees. It is observed that by decreasing the duty cycle by 60%, the droplet diameter increases by 38%, while by doubling the applied magnetic frequency the droplet diameter increases by 38.9%. Moreover, the droplet formation from the nozzle is studied theoretically and numerically, and the results are in good agreement with the experimental data. Using numerical simulation, it is possible to study other dimensionless numbers involved in the problem, which are impossible to study experimentally, which yields to a comprehensive study of the problem. The “beating regime” is observed for the first time in T-junction. The effect of magnetic flux density, duty cycle, applied magnetic frequency, and the continuous-phase flow rate on the ferrofluid droplet formation in T-junction is investigated. Results illustrate that by increasing magnetic flux density, and duty cycle, and decreasing the applied magnetic frequency and the continuous-phase flow rate, the droplet diameter increases. Besides, the droplet diameter and formation frequency under the PWM magnetic field decrease by 25%, and 62%, respectively, compared to those under the DC magnetic field. The regime map of droplet formation and a correlation for non-dimensional diameter based on the dimensionless variables are presented. Finally, the formation, displacement, and mixing of ferrofluid droplets in a digital magnetofluidics platform are investigated. Droplet formation using a surface with a hydrophilic-hydrophobic design inspired by cactus geometry and magnet propulsive force is introduced for the first time. The device can be used as a new method for generating droplets with controlled volume and frequency in digital magnetofluidics. By changing the inlet volumetric flow rate, the distance of the magnet from the hydrophilic section and the hydrophilic section geometry, the possibility of altering the diameter and formation frequency is investigated. Also, the ability of the system for parallel droplet generation is shown linearly and radially. Using the results of this study, more control over the process of droplet formation in the medical and pharmaceutical industries can be achieved
  9. Keywords:
  10. Ferrofluid ; Droplet Formation ; Satellite Droplet ; Drop-on-Demand ; Time-Dependent Magnetic Field

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