This study determines the effect of the configuration of the magnetic field on the movement of gas bubbles that evolve from platinum electrodes. Oxygen and hydrogen bubbles respectively evolve from ...the surface of the anode and cathode and behave differently in the presence of a magnetic field due to their paramagnetic and diamagnetic characteristics. A magnetic field perpendicular to the surface of the horizontal electrode causes the bubbles to revolve. Oxygen and hydrogen bubbles revolve in opposite directions to create a swirling flow and spread the bubbles between the electrodes, which increases conductivity and the effectiveness of electrolysis. For vertical electrodes under the influence of a parallel magnetic field, a horizontal Lorentz force effectively detaches the bubbles and increases the conductivity and the effectiveness of electrolysis. However, if the layout of the electrodes and magnetic field results in upward or downward Lorentz forces that counter the buoyancy force, a sluggish flow in the duct inhibits the movement of the bubbles and decreases the conductivity and the charging performance. The results in this study determine the optimal layout for an electrode and a magnetic field to increase the conductivity and the effectiveness of water electrolysis, which is applicable to various fields including energy conversion, biotechnology, and magnetohydrodynamic thruster used in seawater.
Ferrofluid is a type of smart material consisting of stable colloidal suspensions of magnetic nanoscale particles dispersed in a nonmagnetic carrier liquid. It is known that ferromagnetic ...nanoparticles may improve the magnetic and dielectric properties of host materials, which exhibit novel functionality to provide better microwave absorption properties. A ferrofluid layer subjected to an external magnetic field resulting in cone-shaped structures may further enhance the reflection loss of the incident electromagnetic wave. This study investigates the effect of the ferrofluid crests on the enhancement of the electromagnetic wave shielding effectiveness in the frequency range of 13-18 GHz. It is noted that such a ferrofluid has significant shield effectiveness at the frequency of 16 GHz. When the magnetic field is applied to create the ferrofluid crests, the value of shielding effectiveness (SE) initially increases, then decreases as the field strength is further increased, which causes more area of trough without ferrofluid. The variation of the EMI shield effectiveness is dominated by the arrangement of the ferrofluid crests array, which is closely related to the thickness, composition of the ferrofluid, and external magnetic field condition. The experimental results show that a ferrofluid layer with a thickness of 6 mm subjected to a magnetic flux density of B max =100 mT obtains SE value higher than 21 dB throughout almost all the testing frequency range of 13-18 GHz, and the highest SE max is 29.3 dB locating at 16 GHz.
In comparison to alternative methods for hydrogen production, water electrolysis stands out as the optimal means for obtaining ultra-pure hydrogen. However, its widespread adoption is significantly ...hampered by its low energy efficiency. It has been established that the introduction of an external magnetic field can mitigate energy consumption, consequently enhancing electrolysis efficiency. While much of the research has revealed that an electrode–parallel magnetic field plays a crucial role in enhancing the bubble detachment process, there has been limited exploration of the effect of electrode–normal magnetic fields. In this work, we compare the water electrolysis efficiency of a circular electrode subjected to electrode–normal magnetic field resulting in a magnet edge effect and electrode edge effect by varying the sizes of the magnet and electrode. The findings indicate that a rotational flow caused by the Lorentz force facilitates the detachment of the hydrogen from the electrode surface. However, the rotation direction of hydrogen gas bubbles generated by the magnet edge effect is opposite to that of electrode edge effect. Furthermore, the magnet edge effect has more significant influence on the hydrogen bubbles’ locomotion than the electrode edge effect. With an electrode gap of 30 mm, employing the magnet edge effect generated by a single magnet leads to an average of 4.9% increase in current density. On the other hand, the multiple magnet effects created by multiple small magnets under the electrode can further result in an average 6.6% increase in current density. Nevertheless, at an electrode spacing of 50 mm, neither the magnet edge effect nor the electrode edge effect demonstrates a notable enhancement in conductivity. In reality, the electrode edge effect even leads to a reduction in conductivity.
The dynamics of a flexible micro-swimmer that contains superparamagnetic beads of different diameter in an oscillating field is studied experimentally. Two types of artificial swimmers are fabricated ...to determine the flexing characteristics. The effect of key parameters that dominate the motion of the swimmer is determined. The flexibility initially increases linearly with the frequency and reaches a maximum value at a specific frequency. The field intensity has no significant effect on the flexibility when the swimmer is subjected to a higher oscillating frequency. The instantaneous speed of the swimmer increases almost linearly with the flexibility and reaches a maximum value for a swimmer with a specific geometry. On the other hand, the amplitude of the oscillation has a significant effect on swimming effectiveness. The swimmer achieves a maximum instantaneous speed of 1.78 μm/s to 3.16 μm/s when the oscillating amplitude reaches 3.7–4.2 μm. A flexible swimmer with a moderately high amplitude of oscillation generates effective propulsion in an environment with a low Reynolds number of 2.5×10−6–4.5×10−5.
Magnetic microbeads have recently been the subject of much study because of their potential applications in microfluidic systems which facilitate mixing, labeling, separation, and transport in ...lab-on-a-chip devices. To manipulate the microbeads chain swimming in the low-Reynolds-number environment, various magnetic actuation methods have been employed to obtain the higher propulsive efficiency for the locomotion in the viscous fluid. A flexible flagellum is arguably the simplest mechanism to duplicate as it is a 1-D structure. However, a challenge to create the stable planar beating motion for propulsion generation is to fabricate the magnetic flagellum simultaneously flexible and stable structure at a microscale. Driven by this motive, this paper has constructed a series of artificial flexible flagella composed of self-assembled beads whose flexibility was probed in the influence of an oscillating external field. To effectively use the oscillating magnetic flagellum for the application in the microfluidic system, the measurement of the maximum dimensionless curvature (<inline-formula> <tex-math notation="LaTeX">C_{\mathbf {max}} </tex-math></inline-formula>) and bending rigidity for the flexible microchain are experimentally and theoretically investigated in this paper. At a lower frequency of <inline-formula> <tex-math notation="LaTeX">f=1 </tex-math></inline-formula> Hz, the value of <inline-formula> <tex-math notation="LaTeX">C_{\mathbf {max}} </tex-math></inline-formula> of the flagellum increases linearly with the applied field intensity and gets higher then declines with the increase of the flagellum's length. On the other hand, the longer flagellum has the more stable flexible structure at a higher frequency of <inline-formula> <tex-math notation="LaTeX">f=3 </tex-math></inline-formula>-7 Hz, which resists the amplitude and enhances the deformation of the longer flagellum.
This study uses the effect of flexibility on the propulsive efficiency of swimmers that consist of superparamagnetic particles and which are subjected to an oscillating field to control the movement ...in a low Reynolds number environment. To achieve nonreciprocal motion for a flexible swimmer using a simple and stable structure, two types of artificial flexible swimmers are constructed using self-assembled beads without links and the flexibility and the bending rigidity are investigated under various frequencies. At a low frequency, both the head and the tail oscillate almost synchronously with the field, which leads to a nearly rigid and reciprocal oscillation. The phase angle trajectory for the head significantly leads the tail at a higher frequency of oscillation, which results in a prominent flexible structure and propulsion generation. Furthermore, the flexibility initially increases linearly with the frequency and then reaches the highest value at a specific frequency. The instantaneous velocity of the swimmer almost linearly increases with its flexibility. The most effective oscillating frequency to manipulate the locomotion for the magnetic microbeads swimmer would be at f=7-10 Hz, which resists the amplitude and enhances the flexibility of the microswimmer. Finally, a flexible swimmer associated with a moderate high oscillating amplitude is a favorable configuration for propulsion generation.
A propulsion mechanism for a flexible microswimmer constructed from superparamagnetic microbeads with different diameters and subjected to an oscillating field was studied experimentally and ...theoretically herein. Various types of artificial swimmers with different bending patterns were fabricated to determine the flexibility and an effective waveform for a planar beating flagellum. Waveform evolutions for various swimmer configurations were studied to determine the flexible mechanism of the swimmers. A one-armed microswimmer can propel itself only if the friction of its wavelike body is anisotropic. A swimmer with a larger head and a stronger magnetic dipole moment with a flexible tail allows the bending wave to propagate from the head toward the tail to generate forward thrust. The oscillating head and tail do not simultaneously generate positive thrust all the time within a period of oscillation. To increase the propulsion for a bending swimmer, this study proposes a novel configuration for a microbead swimmer that ensures better swimming efficiency. The ratio of the oscillation amplitude of the head to the length of the swimmer (from 0.26 to 0.28) produces a faster swimmer. On the other hand, the swimmer is propelled more effectively if the ratio of the oscillation amplitude of the tail to the length of the swimmer is from 0.29 to 0.33. This study determined the optimal configuration for a flexible microbead swimmer that generates the greatest propulsion in a low Reynolds number environment.
Water electrolysis is one of the most common methods to produce hydrogen gas with high purity, but its application is limited due to its low energy efficiency. It has been proved that an external ...magnetic field can reduce energy consumption and increase hydrogen production efficiency in water electrolysis. In this study, electrodes with different magnetism were subjected to a perpendicular magnetic field for use in hydrogen production by water electrolysis. Gas bubbles that evolve from the surface of a horizontal electrode detach faster than the bubbles from a vertical electrode. The locomotion of the bubbles is facilitated if the horizontal electrode faces a magnet, which induces the revolution of bubbles between the electrodes. However, the magnetic field does not increase the current density effectively if the electrodes are more than 5 cm apart. A paramagnetic (platinum) electrode has a more significant effect on bubble locomotion than a diamagnetic (graphite) material and is able to increase the efficiency of electrolysis more effectively when a perpendicular magnetic field is applied. The conductivity of platinum electrodes that face a magnet increases if the distance between the electrodes is less than 4 cm, but the conductivity of graphite electrodes does not increase until the inter-electrode distance is reduced to 2 cm. On the other hand, horizontal graphite electrodes that are subjected to a perpendicular magnetic field will generate a higher gas production rate than a platinum electrode without a magnetic field if the inter-electrode distance is less than 1 cm.
A ferrofluid layer separates into numerous subscale crests, which is referred to as Rosensweig instability, whose shape and size depend on the field condition and the composition of the ferrofluid. A ...ferrofluid consisting of nanoscale magnetite particles is also used as an electromagnetic (EM) wave absorption and reflection material. For this study, oil-based and mixture ferrofluid layers that split into various shapes of crests in the presence of an external magnetic field are used to form a protruding structure to reflect and scatter the EM wave and decrease EM radiation energy. For an identical field strength, a mixture ferrofluid layer splits into more crests than an oil-based ferrofluid. A mixture crest shows a less uniform size and shape than the oil-based one. A high-power green laser light is used as a visual EM wave emitting to a crest, which has varying tip angles, and to demonstrate the reflection and scattering. The reflection loss increases as the field strength is increased to create a crest of a smaller tip angle. The reflection loss of an EM wave is significantly affected by the transmitting position on a crest and the shape of a crest. Inter-reflection arises if an EM wave is repeatedly reflected on the surfaces of crests, which contributes to a significant reflection loss. An EM wave incident at an angle of 45° on a crest resulting in a larger area of the inter-reflection zone without specular reflection in a trough gives the most significant reflection loss.
To investigate the use of magnetic microbeads for swimming at low Reynolds number, the flexible structure of microchains comprising superparamagnetic microbeads under the influence of oscillating ...magnetic fields is examined experimentally and theoretically. For a ductile chain, each particle has its own phase angle trajectory and phase-lag angle to the overall field. This present study thoroughly discusses the synchronicity of the local phase angle trajectory between each dyad of beads and the external field. The prominently asynchronous trajectories between the central and outer beads significantly dominate the flexible structure of the oscillating chain. In addition, the dimensionless local Mason number (Mnl) is derived as the solo controlling parameter to evaluate the structure of each dyad of beads in a flexible chain. The evolution of the local Mason number within an oscillating period implies the most unstable position locates near the center of the chain around 0.6P<t<0.8P. Moreover, a chain with a certain length in the influence of the oscillating field would behave the most significant deformation and have the most flexible structure.
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