Experimental and Theoretical Investigation on the Dynamic Response of Ferrofluid Liquid Marbles to Steady and Pulsating Magnetic Fields

Abstract

Liquid marbles are droplets enwrapped by a layer of hydrophobic micro/nanoparticles. Due to the isolation of fluid from its environment, reduction in evaporation rate, low friction with the surfaces, and capability of manipulation even on hydrophilic surfaces, liquid marbles have attracted the attention of researchers in digital microfluidics. This study investigates the manipulation of ferrofluid liquid marbles (FLMs) under DC and pulse width-modulated (PWM) magnetic fields generated by an electromagnet for the first time. At first, the threshold of the magnetic field for manipulating these FLMs is studied. Afterward, the dynamic response of the FLMs to the DC magnetic field for different FLM volumes, coil currents, and initial distances of FLM from the coil is studied, and a theoretical model is proposed. By applying the PWM magnetic field, it is possible to gain more control over the manipulation of the FLMs on the surface and adjust their position more accurately. Results indicate that with a decrease in FLM volume, coil current, and duty cycle, the FLM step length decreases; hence, FLM manipulation is more precise. Under the PWM magnetic field, it is observed that FLM movement is not synchronous with the generated pulse, and even after the coil is turned off, FLMs keep their motion. In the end, with proper adjustment of the electromagnet pulse width, launching of FLMs at a distance farther than the coil is observed.

Dynamic behavior of floating magnetic liquid marbles under steady and pulse-width-modulated magnetic fields

“…Azizian et al 50 proposed a new approach, inspired by billiard ball collisions, for liquid marble manipulation by driving an FM to collide with a non-magnetic LM. Mohammadrashidi et al 51 experimentally investigated the dynamic behavior of ferrofluid liquid marbles on a solid substrate under magnetic fields, in which the LM undergoes discontinuous, less-controllable movement. Since LMs could be controlled on a solid surface or float on the water surface, [15][16][17][18] it has been demonstrated that marbles experience rolling when they are in contact with a solid surface, whereas floating marbles could undergo sliding motions, which is the preferred operating situation in DMF.…”

“…Unlike naked ferrofluid droplets, manipulating biocompatible LMs such as LMMSs is not fully understood, especially when they move on the water surface driven by a time-varying magnetic field. In this regard, FMs have attracted the attention of researchers more than LMMSs; 28,46,51 however, LMMSs could be preferable based on their higher biocompatibility, as magnetic nanoparticles and surfactants are in less contact with the biological cargo in the core fluid. 29,[54][55][56] The integration of electromagnets with a pulse-widthmodulation (PWM) circuit is promising as a stimulus for the manipulation of floating magnetic LMs.…”

Switchable Wettability States of a Ferrofluid Droplet atop a Hydrophobic Interface

“…While such “smart” machines can be actuated by different stimuli, magnetic fields appear to be particularly attractive for their precise controllability and efficacy in realizing a wide variety of shapes and functionalities. , The most common variant of such a micromachine can simply be a liquid drop made of a suspension of magnetic nanoparticles commonly known as ferrofluids. Despite being elusively simple to synthesize, the ferrofluids offer great promise in a wide variety of automated applications ranging from liquid cargo delivery, − cell and tissue engineering, , water purification to on-chip mixing of liquid samples . Although their different shape modulations, splitting, and merging were successfully demonstrated in previous works, the proposition of tuning their wettability at will, preferably in a programmable manner, continues to offer an outstanding challenge when the applied field strength exceeds the saturation magnetization limit.…”

Vibration and Jumping of Ferrofluid Marbles under an Initial Magnetic Perturbation

“…Mohammadrashidi et al theoretically and experimentally studied the motion dynamics of ferrofluid marbles under steady and unsteady magnetic fields. They also observed the launching motion of ferrofluid marble by applying a single-pulse magnetic field . Azizian et al studied the collision of a ferrofluid marble with a water marble and examined the potential of using this collision for manipulating non-magnetic liquid marbles .…”

“…They also observed the launching motion of ferrofluid marble by applying a single-pulse magnetic field. 66 Azizian et al studied the collision of a ferrofluid marble with a water marble and examined the potential of using this collision for manipulating non-magnetic liquid marbles. 67 Yang et al investigated the reciprocating motion of ferrofluid marbles floating on the water surface.…”