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E-book Magnetic Hybrid-Materials : Multi-scale Modelling, Synthesis, and Applications
The design of hybrid materials that can be manipulated by magnetic fields is possibleby incorporating inorganic magnetic nanoobjects into complex soft matrices such asgels [1], elastomers [2], liquid crystals [3], or biologicalfluids [4] in a predeterminedway. A proper setup allows the application of external magneticfields to either analyzeor alter the material properties of such systems on a nanoscopic scale. The resultinghybrid structures have promising responsive properties that can be categorized intoapplications of interest either for actuation or sensing.The fascinating prospective arising from magnetic nanoparticles dispersed inliquids (ferrofluids) [5] and the manipulation of their structure [6] and dynamics [7] bymagneticfields have led to a broadfield of applications. On the other side of thematerial spectrum, magnetoactive elastomers attract increasing attention due to thetheoretical prediction and experimental observation of even more sophisticated effectssuch as magnetoresponsive shape changes or mechanical properties, as often observedfor elastomers [8–10]. The magnetic probe particles are further useful nanoscopic magnetic tracers,allowing the investigation of the nanoparticle interaction with their surroundingmatrix by using dynamic magnetic methods on small sample volumes in a nonde-structive way [11]. Of particular interest is the option to investigate the mechanicalproperties of soft matter at the length scale of the probe particle, giving access to localinformation in microstructured samples that are hardly accessible by classical methods[12, 13].Research of this kind strongly benefits from new developments in the controlledsynthesis of prospective tracer particles with improved uniformity, stability, controlledsurface characteristics, and novel coupling mechanisms [10, 14–16]. In addition, thedevelopment and availability of new and better resolved methods for the investigationof structure and dynamics in such soft matter hybrid systems facilitates the detectionand establishment of novel coupling measuring modes [17]. Recent studies demon-strate that by variation of the particle characteristics in terms of size [18], shape [19],surface functionality [20], and magnetic behavior [21], the interaction between theprobe or actuator particles and their environment can be tailored in wide limits. The flow properties of complex fluids, as generally investigated by rheology, arerelevant for the processing and application properties of all kinds of products in dailylife, such as cosmetics and food. A broad spectrum of rheological methods is availablefor their assessment, spanning a large range of time and size regimes as well asmeasuring geometries.The employment of microscopic approaches for the investigation of complex fluidsis often related to the application of small tips or particles as probes. While passivemethods, like particle tracking microscopy [22], diffusing wave spectroscopy [23], orfluorescence correlation spectroscopy [24], have been shown to yield good results formany materials, active methods where the probes are exposed to active displacement,generally benefit from a broader frequency range and higher possible torques enablingthe analysis of even elastic samples [25]. Already in 1950 Crick et al. showed that themicrorheological approach has significant benefits for the analysis of biologicalsamples. As low sample volume is needed, the mechanical stress subjected to thesample is low, and a wide frequency range is accessible [26]. The original idea of thistechnique is to obtain macrorheological results on small samples in a nondestructiveway. While this is fulfilled for many relevant samples, an analogous approach usingprobe particles in a size more similar to the relevant length scales of the matrix materialopens even the pathway to the investigation of size-dependent properties, as relevantfor internal dynamics in systems such as nanocomposites and cells [27, 28]
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