Tuning cracks by exploiting the shape of particles and external magnetic field

Drying of a colloidal dispersion usually leads to the formation of particulate film with random cracks. The cracks in particulate film can have periodic arrangement with tuneable spacing and are known to be useful for practical applications such as for fabrication of lithographic templates and nano-channels. Various methodology has been adopted to generate the parallel and ordered cracks, the common one is via applying an external field such as magnetic field or electric field. We report here the controlled manipulation of crack orientation for colloidal films consisting of magnetically active particle (hematite ellipsoids), using an external magnetic field. Drying sessile drop experiments are performed in the presence and absence of magnetic field and a coffee ring like particle deposits are observed. The dried region consists of circular cracks in the absence of field while linear cracks (along the chord of the ring) in the presence of field. Moreover, we found that the crack orientations can be systematically altered by tuning magnetic field strength. We conjecture that the competition between the hydrodynamic torque and magnetic torque experienced by the particles during the drying of colloidal dispersion decides the final orientation of the particles and the cracks. The alteration of crack direction by controlling the orientation of ellipsoids in the particulate films by application of magnetic field is presented in detail. Please click Additional Files below to see the full abstract

Rapidly Solidified Rare-Earth Permanent Magnets: Processing, Properties, and Applications

Rapidly solidified rare-earth-based permanent magnets are considered to have better potential as permanent magnets compared to the conventional bulk materials, which can be attributed to their improved microstructure and better magnetic properties compared to rare-earth magnets synthesized by the conventional (powder metallurgy) routes. The performance (quality) of these magnets depends on the thermodynamics and kinetics of the different processing routes, such as atomization, melt spinning, and melt extraction. Here, we review the various processing routes of rapidly solidified rare-earth permanent magnets and the related properties and applications. In the review, some specific alloy systems, such as Sm–Co-based alloys, Nd–Fe–B, and interstitially modified Fe-rich rare-earth magnets are discussed in detail mentioning their processing routes and subsequently achieved crystal structure, microstructure and magnetic properties, and the related scopes for various applications. Some newly developed nanocomposites and thin-film magnets are also included in the discussion

Self-assembly of particles via controlled evaporation

Evaporation of solvent from a dispersion is a simple and effective method to direct the self-assembly of colloidal scale materials. In particular, the drying of particle laden sessile drops has received considerable attention since pioneering work in the late 1990’s. Upon evaporation, suspension drops leave a distinct ring-like deposit of particles at the periphery of the drop. It is widely accepted that physics of pattern formation in drying of drops containing in-soluble material is identical to that observed in drying of a coffee drop. Both the formation and suppression of coffee-stains are fundamentally and technologically important. There is need for the design of strategies to prevent coffee stains, which are unwanted in several applications. However, there are several reports where the desirable consequences of coffee-stain formation are exploited – especially in the field of separation technology and in the detection and diagnosis of diseases. Please click Additional Files below to see the full abstract

Magnetic Proximity Effect in YBa₂Cu₃O₇/La_2/3Ca_1/3MnO₃ and YBa₂Cu₃O₇/LaMnO₃₊ Superlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa₂Cu₃O₇ and ferromagnetic-metallic La0.67Ca0.33MnO₃ or ferromagnetic-insulating LaMnO₃₊. We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO₃ and almost absent for ferromagnetic-insulating LaMnO₃₊. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature

Rapidly Solidified Rare-Earth Permanent Magnets: Processing, Properties, and Applications

Rapidly solidified rare-earth-based permanent magnets are considered to have better potential as permanent magnets compared to the conventional bulk materials, which can be attributed to their improved microstructure and better magnetic properties compared to rare-earth magnets synthesized by the conventional (powder metallurgy) routes. The performance (quality) of these magnets depends on the thermodynamics and kinetics of the different processing routes, such as atomization, melt spinning, and melt extraction. Here, we review the various processing routes of rapidly solidified rare-earth permanent magnets and the related properties and applications. In the review, some specific alloy systems, such as Sm–Co-based alloys, Nd–Fe–B, and interstitially modified Fe-rich rare-earth magnets are discussed in detail mentioning their processing routes and subsequently achieved crystal structure, microstructure and magnetic properties, and the related scopes for various applications. Some newly developed nanocomposites and thin-film magnets are also included in the discussion

Surface-specific ordering of reverse micelles in confinement

We have applied holographic X-ray diffraction from fluid-filled channel arrays for model-independent density reconstruction of spherical AOT/water/isooctane reverse micelles (average diameter σ***Missing image substitution***12–13 nm) confined between planar surfaces. We find the confinement-induced ordering of the reverse micelles to strongly depend on the surface potential of the confining surfaces: for hydrophilic surfaces we find diffuse monolayers centered at 13 ± 3 nm away from the solid–fluid interface, while for hydrophobic surfaces we observe close-packed monolayers at the solid–fluid interface

Synergy between the crack pattern and substrate elasticity in colloidal deposits

Desiccation cracks in colloidal deposits occur to release the excess strain energy arising from the competition between the drying induced shrinkage of the deposit and its adhesion to the substrate. Here we report remarkably different morphology of desiccation cracks in the dried patterns formed by the evaporation of sessile drops containing colloids on elastomer (soft) or glass (stiff) substrates. The change in the crack pattern, i.e., from radial cracks on stiff substrates to circular cracks on soft substrates, is shown to arise solely due to the variation in elasticity of the underlying substrates. Our experiments and calculations reveal an intricate correlation between the desiccation crack patterns and the substrate's elasticity. The mismatch in modulus of elasticity between the substrate and that of the particulate deposit significantly alters the energy release rate during the nucleation and propagation of cracks. The stark variation in crack morphology is attributed to the tensile or compressive nature of the drying-induced in-plane stresses

Molecular liquid under nanometre confinement: density profiles underlying oscillatory forces

Ultrathin (<12 nm) films of tetrakis(trimethyl)siloxysilane (TTMSS) have been confined by atomically flat mica membranes in the presence and absence of applied normal forces. When applying normal forces, discrete film thickness transitions occur, each involving the expulsion of TTMSS molecules. Using optical interferometry we have measured the step size associated with a film thickness transition (7.5 Å for compressed, 8.4 Å for equilibrated films) to be smaller than the molecular diameter of 9.0 Å. Layering transitions with a discrete step size are commonly regarded as evidence for strong layering of the liquid's molecules in planes parallel to the confining surfaces and it is assumed that the layer spacing equals the measured periodicity of the oscillatory force profile. Using x-ray reflectivity (XRR), which directly yields the liquid's density profile along the confinement direction, we show that the layer spacing (10–11 Å) proves to be on average significantly larger than both the step size of a layering transition and the molecular diameter. We observe at least one boundary layer of different electron density and periodicity than the layers away from the surfaces