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Experimental Investigation of Dynamics of Ferrofluid-Based Liquid Marbles Under External Magnetic Fields

Mohammad Rashidi, Mahbod | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55881 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Shafii, Mohammad Behshad; Taghipoor, Mojtaba; Bijarchi, Mohamad Ali
  7. Abstract:
  8. Liquid marbles are droplets enwrapped by a layer of micro/nanoparticles. Due to the insulation of inner fluid from the environment, the lower evaporation rate of the fluid, and the capability of motion even on hydrophilic surfaces, these liquid marbles, as an alternative to conventional droplets, have attracted the attention of researchers in digital microfluidics. Utilizing ferrofluid as the liquid core of the liquid marbles enables remote control over the liquid marbles by an external magnetic field. In this research, the dynamic behavior of ferrofluid liquid marbles under the magnetic field generated by an electromagnet is investigated. In this research, governing physics on the motion of these liquid marbles is studied. First, the threshold of the magnetic field for the motion of liquid marbles is experimentally and theoretically investigated. Then, the dynamic response of liquid marbles to a DC magnetic field is studied, and effective parameters on the motion of these liquid marbles, namely liquid marbles volume, electromagnet current, and the initial distance of liquid marbles from the electromagnet are experimentally and theoretically examined. Then, the capability of manipulating these liquid marbles using a pulse-width-modulated magnetic field is examined. It is observed that under an alternating magnetic field, the motion of liquid marbles is not synchronous with the generated magnetic field, and even during the electromagnet inactive time intervals, liquid marbles keep their motion due to their high inertia and low friction with the surface. Afterward, by applying a single-pulse magnetic field, a novel method for the launching of liquid marbles to distances farther than the electromagnet position is proposed. Four different regimes for the liquid marble launching phenomena are detected, and conditions in which these regimes take place are investigated. It is observed that by increasing the electromagnet pulse width, four regimes of partial launching, full launching, backward launching, and full stop occur, respectively. Afterward, the oscillating response of the liquid marbles under an initial magnetic stimulus is experimentally and theoretically studied. By applying a magnetic force to the liquid marbles by an electromagnet, liquid marbles deform, and when the magnetic field is terminated, liquid marbles return to their final equilibrium state after a damped oscillating motion. A linear mass-spring-damper model is proposed to describe the free oscillations of liquid marbles, and then, the effect of liquid marbles volume and initial electromagnet current on the characteristics of these oscillations, more specifically, natural frequency, damping ratio, and deformations of liquid marble during the oscillation are investigated. Plus, by the analysis of the frequency response of the liquid marbles, their effective surface tension is measured. Moreover, the coalescence process of two liquid marbles under the external magnetic field is experimentally studied, and it is seen that two liquid marbles coalesce providing the magnetic force is strong enough that the direct contact between the liquid cores of the liquid marbles is established. Results indicate that by increment in liquid marbles volume, a lower magnetic field is required for the liquid marbles to coalesce. In addition, it is observed that the required magnetic field for the coalescence of two liquid marbles, both of which are formed by the coalescence of two smaller liquid marbles, is higher than that for two liquid marbles originally formed with those volumes. Moreover, the required magnetic field for the coalescence of two different-volume liquid marbles is mainly a function of the smaller liquid marble volume
  9. Keywords:
  10. Coalescence ; Ferrofluid ; Digital Microfluidics ; Oscillations ; Magnetic Manipulation ; Liquid Marbles

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