Ferro-hydrodynamic interactions between ferrofluid droplet pairs in simple shear flows

Hassan, Rifat; Wang, Cheng

doi:10.1016/j.colsurfa.2020.124906

Cited by 13 publications

(13 citation statements)

References 63 publications

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“…Moreover, Han et al conducted a three-dimensional numerical study on the effects of magnetohydrodynamics (MHD) on the spreading characteristics of a liquid metal droplet on a flat solid surface and observed an inhibition effect on the spreading dynamics under horizontal magnetic fields. Additionally, we have demonstrated versatile manipulation of droplets using magnetic fields in our previous works. − …”

Section: Introductionmentioning

confidence: 94%

“…Multiphase flows are one of the most common phenomena that occur both in nature and industrial processes, − which often involve severe topological changes on the interface during the transport mechanism, including deformation, migration, , breakup, and coalescence of the interface. As a result, a diffuse-interface approach-based phase-field (PF) method is employed in the numerical simulations to track the free interface of the droplet.…”

Section: Computational Algorithmmentioning

confidence: 99%

“…In addition to hydrodynamic forces, droplet manipulation can be achieved through the utilization of external forces, i.e., electric , or magnetic fields. − However, to attain magnetic control, either the continuous phase or droplet phase needs to be a ferrofluida fluid with magnetic properties, which due to its unique response to magnetic fields has gained immense popularity in numerous applications, including cancer treatment, sealing rotating shafts, and heat exchangers . Also, since ferrofluid offers greater flexibility in operations, it shows huge promise in a 3D metal printing process where the quality of printing, followed by shape distortion of metal, is defined by the overall spreading dynamics of the droplet during laser operation .…”

Section: Introductionmentioning

confidence: 99%

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Spreading Dynamics of an Impinging Ferrofluid Droplet on Hydrophilic Surfaces under Uniform Magnetic Fields

Hassan

Wang

2021

Langmuir

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This paper reports a numerical investigation on the spreading dynamics of an impinging ferrofluid droplet on solid hydrophilic surfaces (i.e., θw ≤ 60°) in the presence of uniform magnetic fields. A finite element method-based commercial solver is implemented to perform several numerical simulations, which uses a phase-field (PF) method to couple both the flow and magnetic fields. The results demonstrate that a uniform magnetic field is capable of controlling the spreading dynamics of an impinging droplet on hydrophilic substrates. Additionally, the application of a magnetic field results in the generation of a steady-state droplet shape with a reduced base diameter and an increased apex height at higher magnetic Bond numbers at the end of the spreading process. Moreover, as the viscosity of the droplet decreases, the droplet experiences an increase in its primary spreading diameter, which can be even reduced through the implementation of a vertical uniform magnetic field. Additionally, an oscillatory motion appears in a droplet during the spreading phenomenon at lower Ohnesorge numbers (i.e., Oh = 0.023), which is further sustained for a longer period of time in the relaxation phase with increased amplitudes in the case of an extremely low-viscosity droplet (i.e., Oh = 0.002) before attaining a final equilibrium shape. Furthermore, at Oh = 0.002, the droplet undergoes a breakup event after the impact for a short period of time, while the magnetic field induces an elastic behavior in a droplet at lower viscosities (i.e., Oh = 0.023) during the free fall under gravity.

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Section: Introductionmentioning

confidence: 94%

Section: Computational Algorithmmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Spreading Dynamics of an Impinging Ferrofluid Droplet on Hydrophilic Surfaces under Uniform Magnetic Fields

Hassan

Wang

2021

Langmuir

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“…Ferrofluids are another examples of a liquid magnet, which contain colloidal suspensions of magnetic nanoparticles in a carrier fluid and experience deformation under the exposure to magnetic fields. − Also, due to the uniform distribution of magnetic nanoparticles, ferrofluids can be transported to different locations for liquid phase reactions and are conventionally used as a sealant in hard drives and a coolant in loudspeakers . Nguyen et al conducted experiments on the motion of a droplet under a moving permanent magnetic field and derived a critical velocity, above which the droplet fails to move with the same velocity as the moving magnet.…”

Section: Introductionmentioning

confidence: 99%

“…However, the current numerical studies in the literature only focused on larger-sized droplets, , so that gravitational forces can induce coalescence between droplets that are initially placed in contact with each other or droplets on patterned surfaces. , Moreover, as per our knowledge, till now, no one has ever performed numerical analyses on the dynamic behavior of micrometer-sized droplets (where gravitational forces are negligible in compared to viscous forces) on an open surface under the presence of magnetic fields that will be useful to a wide range of microfluidics applications, including biological and chemical assays. ,,− As a result, in this article, we numerically investigate the dynamics of a pair of sessile droplets on a hydrophobic surface under the effect of a permanent magnetic field, which ultimately leads to a coalescence-induced jumping/no jumping phenomenon under certain operating conditions. Here, a 2D computational scheme is adopted, which is widely proven to capture the dynamic evolution of the droplet interface with great precision, while minimizing the use of a considerable amount of computational resources, − which further facilitates in the investigation on the effects of a wide range of parameters, that is, magnetic field strength, contact angle, and viscosity ratio on the dynamics of droplets on hydrophobic surfaces.…”

Section: Introductionmentioning

confidence: 99%

Digital Microfluidics: Magnetic Transportation and Coalescence of Sessile Droplets on Hydrophobic Surfaces

2021

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Magnetic digital microfluidics is advantageous over other existing droplet manipulation methods, which exploits magnetic forces for actuation and offers the flexibility of implementation in resource-limited point-of-care applications. This article discusses the dynamic behavior of a pair of sessile droplets on a hydrophobic surface under the presence of a permanent magnetic field. A phase field method-based solver is employed in a two-dimensional computational domain to numerically capture the dynamic evolution of the droplet interfaces, which again simultaneously solves the magnetic and flow fields. On a superhydrophobic surface (i.e., θc = 150°), the nonuniform magnetic field forces the pair of sessile droplets to move toward each other, which eventually leads to a jumping off phenomenon of the merged droplet from the solid surface after coalescence. Also, there exists a critical magnetic Bond number Bo m cr, beyond which no coalescence event between droplets is observed. Moreover, on a less hydrophobic surface (θc ≤ 120°), the droplets still coalesce under a magnetic field, although the merged droplet does not experience any upward flight after coalescence. Also, the merging phenomenon at lower contact angle values (i.e., θc = 90°) appears significantly different than at higher contact angle values (i.e., θc = 120°). Additionally, if the pair of sessile droplets is dispersed to a different surrounding medium, the viscosity ratio plays a significant role in the upward flight of the merged droplet, where the coalesced droplet exhibits increased vertical migration at higher viscosity ratios.

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Horizontal and Vertical Coalescent Microrobotic Collectives Using Ferrofluid Droplets

et al. 2023

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Many artificial miniature robotic collectives have been developed to overcome the inherent limitations of inadequate individual capabilities. However, the basic building blocks of the reported collectives are mainly in the solid state, where the morphological boundaries of internal individuals are clear and cannot genuinely merge. Miniature robotic collectives based on liquid units still need to be explored; such on‐demand mergeable swarm systems are advantageous for adapting to the changing external environment. Here, a strategy to achieve a coalescent collective system we presented that exploits the ferrofluid droplets' splitting and coalescence properties to trigger the formation of horizontal multimodal and vertical gravity‐resistant collectives and unveil pattern‐enabled robotic functionalities. When subjected to a time‐varying magnetic field, the droplet swarm exhibits a variety of morphologies ranging from horizontal collectives, including vortex‐like, chain‐like, and crystal‐like patterns to vertical layer‐upon‐layer patterns. Using experiments and simulations, the formation and transformation of different morphological collectives are shown and their robust environmental adaptability are demonstrated. Potential applications of the multimodal droplet collectives are presented, including exploring an unknown environment, targeted object delivery, and fluid flow filtration in a lab‐on‐a‐chip. This work may facilitate the design of microrobotic swarm systems and expand the range of materials for miniature robots.

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Ferro-hydrodynamic interactions between ferrofluid droplet pairs in simple shear flows

Cited by 13 publications

References 63 publications

Spreading Dynamics of an Impinging Ferrofluid Droplet on Hydrophilic Surfaces under Uniform Magnetic Fields

Spreading Dynamics of an Impinging Ferrofluid Droplet on Hydrophilic Surfaces under Uniform Magnetic Fields

Digital Microfluidics: Magnetic Transportation and Coalescence of Sessile Droplets on Hydrophobic Surfaces

Horizontal and Vertical Coalescent Microrobotic Collectives Using Ferrofluid Droplets

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