Features of magnetorheology of biocompatible chain-forming ferrofluids with multi-core magnetic nanoparticles: Experiment and simulation

Comprehensive Analysis of the Structural Evolution and Dynamic Mechanical Behavior of Ferrofluids through Coarse-Grained Molecular Dynamics and Experimental Testing

Understanding and predicting the relationship between the dynamic structure and dynamic mechanical behavior of ferrofluids at the mesoscale presents a significant challenge. Therefore, in this study, a ferrofluid model that considered the molecular structure of the carrier liquid was constructed to perform coarse-grained molecular dynamics simulations of the mesoscale structural evolution and internal particle arrangement characteristics of ferrofluids under the influence of a magnetic field. Subsequently, oscillatory shear deformation was applied to further investigate the mechanical properties of ferrofluids under dynamic strain, as well as the deformation and disruption of their orientational structures. The simulations show that the columnar aggregation of magnetic particles imparts typical viscoelastic characteristics to the ferrofluid, and these aggregated structures gradually deform and break down as the strain amplitude increases. During dynamic oscillatory shear deformation, the enhancement of the magnetic field allows the aggregated structure of magnetic particles to exhibit better resistance to deformation, thereby improving the absorption and dissipation of mechanical energy. The dynamic mechanical properties of ferrofluids obtained from coarse-grained simulations closely align with the experimental results under moderate to high magnetic fields, allowing for a certain degree of predictive capability regarding the dynamic mechanical behavior of ferrofluids.

Dynamically Assembling Magnetic Nanochains as New Generation of Swarm-Type Magneto-Mechanical Nanorobots Affecting Biofilm Integrity

Bacterial resistance is gaining ground and novel, unconventional strategies are required to improve antibiotic treatments. As a synthetic analog of planktonic bacilli, the natural bacterial swimmers that can penetrate bacterial biofilms, ultra-short propelling magnetic nanochains are presented as bioinspired magnetic nanorobots, enhancing the antibiotic treatment in biofilm-forming Staphylococcus epidermidis. Propelling nanochains, activated by a low intensity (<20 mT) and low frequency (<10 Hz) rotating magnetic field (RMF), prompt the otherwise resistant biofilm-forming bacteria to become sensitive to methicillin, resulting in the killing of 99.99% of bacteria. While magnetic force-driven spherical magnetic nanoparticles were previously reported as unidirectional biofilm channel diggers, propelling nanochains emerge as second-generation magnetic nanorobots, which, due to their magnetic core, shape anisotropy, and negative zeta potential, combine magnetic responsiveness, torque-driven movement, and attractive electrostatic interactions to attach to bacterial aggregates and multi-directionally protrude throughout the biofilm, indulging mechanical forces. These synergistic effects, in combination with an antibiotic drug, destroy the bacterial extracellular matrix and eradicate the formed biofilm, as confirmed with several complementary techniques.

Iron oxide magnetic aggregates: Aspects of synthesis, computational approaches and applications

Superparamagnetic magnetite nanoparticles have been central to numerous investigations in the past few decades for their use in many applications, such as drug delivery, medical diagnostics, magnetic separation, and material science. However, the properties of single magnetic nanoparticles are sometimes not sufficient to accomplish tasks where a strong magnetic response is required. In light of this, aggregated magnetite nanoparticles have been proposed as an alternative advanced material, which may expand and combine some of the advantages of single magnetic nanoparticles, including superparamagnetism, with an enhanced magnetic moment and increased colloidal stability. This review comprehensively discusses the current literature on aggregates made of magnetic iron oxide nanoparticles. This review is divided into three sections. First, the current synthetic strategies for magnetite nanoparticle aggregates are discussed, together with the influence of different stabilizers on the primary crystals and the final aggregate size and morphology. The second section is dedicated to computational approaches, such as density functional methods (which permit accurate predictions of electronic and magnetic properties and shed light on the behavior of surfactant molecules on iron oxide surfaces) and molecular dynamics simulations (which provide additional insight into the influence of ligands on the surface chemistry of iron oxide nanocrystals). The last section discusses current and possible future applications of iron oxide magnetic aggregates, including wastewater treatment, water purification, medical applications, and magnetic aggregates for materials displaying structural colors.

Field-Induced Agglomerations of Polyethylene-Glycol-Functionalized Nanoclusters: Rheological Behaviour and Optical Microscopy

This research aims to investigate the agglomeration processes of magnetoresponsive functionalized nanocluster suspensions in a magnetic field, as well as how these structures impact the behaviour of these suspensions in biomedical applications. The synthesis, shape, colloidal stability, and magnetic characteristics of PEG-functionalized nanoclusters are described in this paper. Experiments using TEM, XPS, dynamic light scattering (DLS), VSM, and optical microscopy were performed to study chain-like agglomeration production and its influence on colloidal behaviour in physiologically relevant suspensions. The applied magnetic field aligns the magnetic moments of the nanoclusters. It provides an attraction between neighbouring particles, resulting in the formation of chains, linear aggregates, or agglomerates of clusters aligned along the applied field direction. Optical microscopy has been used to observe the creation of these aligned linear formations. The design of chain-like structures can cause considerable changes in the characteristics of ferrofluids, ranging from rheological differences to colloidal stability changes.

Correlating the Dipolar Interactions Induced Magneto-Viscoelasticity and Thermal Conductivity Enhancements in Nanomagnetic Fluids

The effective thermal management of electronic system holds the key to maximize their performance. The recent miniaturization trends require a cooling system with high heat flux capacity, localized cooling, and active control. Nanomagnetic fluids (NMFs) based cooling systems have the ability to meet the current demand of the cooling system for the miniaturized electronic system. However, the thermal characteristics of NMFs have a long way to go before the internal mechanisms are well understood. This review mainly focuses on the three aspects to establish a correlation between the thermal and rheological properties of the NMFs. First, the background, stability, and factors affecting the properties of the NMFs are discussed. Second, the ferrohydrodynamic equations are introduced for the NMFs to explain the rheological behavior and relaxation mechanism. Finally, different theoretical and experimental models are summarized that explain the thermal characteristics of the NMFs. Thermal characteristics of the NMFs are significantly affected by the morphology and composition of the magnetic nanoparticles (MNPs) in NMFs as well as the type of carrier liquids and surface functionalization that also influences the rheological properties. Thus, understanding the correlation between the thermal characteristics of the NMFs and rheological properties helps develop cooling systems with improved performance.

Multicore-based ferrofluids in zero field: initial magnetic susceptibility and self-assembly mechanisms

The necessity to improve magnetic building blocks in magnetic nano-structured soft materials stems from a fascinating potential these materials have in bio-medical applications and nanofluidics. Along with practical reasons, the interplay of magnetic and steric interactions on one hand, and entropy, on the other, makes magnetic soft matter fundamentally challenging. Recently, in order to tailor magnetic response of the magnetic particle suspensions, the idea arose to replace standard single-core nanoparticles with nano-sized clusters of single-domain nanoparticles (grains) rigidly bound together by solid polymer matrix - multicore magnetic nanoparticles (MMNPs). To pursue this idea, a profound understanding of the MMNP interactions and self-assembly is required. In this work we present a computational study of the MMNP suspensions and elucidate their self-assembly and magnetic susceptibility. We show that depending on the magnetic moment of individual grains the suspensions exhibit qualitatively distinct regimes. Firstly, if the grains are moderately interacting, they contribute to a significant decrease of the remanent magnetisation of MMNPs and as such to a decrease of the magnetic susceptibility, this way confirming previous findings. If the grains are strongly interacting, instead, they serve as anchor points and support formation of grain clusters that span through several MMNPs, leading to MMNP cluster formation and a drastic increase of the initial magnetic response. Both the topology of the clusters and their size distribution in MMNP suspensions is found to be notably different from those formed in conventional magnetic fluids or magnetorheological suspensions.

Magnetic Nanoparticles: Current Advances in Nanomedicine, Drug Delivery and MRI

Magnetic nanoparticles (MNPs) have evolved tremendously during recent years, in part due to the rapid expansion of nanotechnology and to their active magnetic core with a high surface-to-volume ratio, while their surface functionalization opened the door to a plethora of drug, gene and bioactive molecule immobilization. Taming the high reactivity of the magnetic core was achieved by various functionalization techniques, producing MNPs tailored for the diagnosis and treatment of cardiovascular or neurological disease, tumors and cancer. Superparamagnetic iron oxide nanoparticles (SPIONs) are established at the core of drug-delivery systems and could act as efficient agents for MFH (magnetic fluid hyperthermia). Depending on the functionalization molecule and intrinsic morphological features, MNPs now cover a broad scope which the current review aims to overview. Considering the exponential expansion of the field, the current review will be limited to roughly the past three years.

Ferrofluids and bio-ferrofluids: looking back and stepping forward

Ferrofluids investigated along for about five decades are ultrastable colloidal suspensions of magnetic nanoparticles, which manifest simultaneously fluid and magnetic properties. Their magnetically controllable and tunable feature proved to be from the beginning an extremely fertile ground for a wide range of engineering applications. More recently, biocompatible ferrofluids attracted huge interest and produced a considerable increase of the applicative potential in nanomedicine, biotechnology and environmental protection. This paper offers a brief overview of the most relevant early results and a comprehensive description of recent achievements in ferrofluid synthesis, advanced characterization, as well as the governing equations of ferrohydrodynamics, the most important interfacial phenomena and the flow properties. Finally, it provides an overview of recent advances in tunable and adaptive multifunctional materials derived from ferrofluids and a detailed presentation of the recent progress of applications in the field of sensors and actuators, ferrofluid-driven assembly and manipulation, droplet technology, including droplet generation and control, mechanical actuation, liquid computing and robotics.

Optothermal grid activation of microflow with magnetic nanoparticle thermophoresis for microfluidics

We report focused light-induced activation of intense magnetic microconvection mediated by suspended magnetic nanoparticles in microscale two-dimensional optothermal grids. Fully anisotropic control of microflow and mass transport fluxes is achieved by engaging the magnetic field along one or the other preferred directions. The effect is based on the recently described thermal diffusion-magnetomechanical coupling in synthetic magnetic nanofluids. We expect that the new phenomenon can be applied as an efficient all-optical mixing strategy in integrated microfluidic devices. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Bioinspired Magnetic Nanochains for Medicine

Superparamagnetic iron oxide nanoparticles (SPIONs) have been widely used for medicine, both in therapy and diagnosis. Their guided assembly into anisotropic structures, such as nanochains, has recently opened new research avenues; for instance, targeted drug delivery. Interestingly, magnetic nanochains do occur in nature, and they are thought to be involved in the navigation and geographic orientation of a variety of animals and bacteria, although many open questions on their formation and functioning remain. In this review, we will analyze what is known about the natural formation of magnetic nanochains, as well as the synthetic protocols to produce them in the laboratory, to conclude with an overview of medical applications and an outlook on future opportunities in this exciting research field.

Long-Range Ordering Effects in Magnetic Nanoparticles

The challenge for synthesizing magnetic nanoparticle chains may be achieved under the application of fixation fields, which are the externally applied fields, enhancing collective magnetic features due to adequate control of dipolar interactions among magnetic nanoparticles. However, relatively little attention has been devoted to how size, concentration of magnetic nanoparticles, and intensity of an external magnetic field affect the evolution of chain structures and collective magnetic features. Here, iron oxide nanoparticles are developed by the coprecipitation method at diameters below (10 and 20 nm) and above (50 and 80 nm) their superparamagnetic limit (at about 25 nm) and then are subjected to a tunable fixation field (40-400 mT). Eventually, the fixation field dictates smaller particles to form chain structures in two steps, first forming clusters and then guiding chain formation via "cluster-cluster" interactions, whereas larger particles readily form chains via "particle-particle" interactions. In both cases, dipolar interactions between the neighboring nanoparticles augment, leading to a substantial increase in their collective magnetic features which in turn results in magnetic particle hyperthermia efficiency enhancement of up to one order of magnitude. This study provides new perspectives for magnetic nanoparticles by arranging them in chain formulations as enhanced performance magnetic actors in magnetically driven magnetic applications.

Chain Formation and Phase Separation in Ferrofluids: The Influence on Viscous Properties

Ferrofluids have attracted considerable interest from researchers and engineers due to their rich set of unique physical properties that are valuable for many industrial and biomedical applications. Many phenomena and features of ferrofluids' behavior are determined by internal structural transformations in the ensembles of particles, which occur due to the magnetic interaction between the particles. An applied magnetic field induces formations, such as linear chains and bulk columns, that become elongated along the field. In turn, these structures dramatically change the rheological and other physical properties of these fluids. A deep and clear understanding of the main features and laws of the transformations is necessary for the understanding and explanation of the macroscopic properties and behavior of ferrofluids. In this paper, we present an overview of experimental and theoretical works on the internal transformations in these systems, as well as on the effect of the internal structures on the rheological effects in the fluids.