Micrometric Monodisperse Solid Foams as Complete Photonic Bandgap Materials

Solid foams with micrometric pores are used in different fields (filtering, 3D cell culture, etc.), but today, controlling their foam geometry at the pore level, their internal structure, and the monodispersity, along with their mechanical properties, is still a challenge. Existing attempts to create such foams suffer either from slow speed or size limitations (above 80 μm). In this work, by using a temperature-regulated microfluidic process, 3D solid foams with highly monodisperse open pores (PDI lower than 5%), with sizes ranging from 5 to 400 μm and stiffnesses spanning 2 orders of magnitude, are created for the first time. These features open the way for exciting applications, in cell culture, filtering, optics, etc. Here, the focus is set on photonics. Numerically, these foams are shown to open a 3D complete photonic bandgap, with a critical index of 2.80, thus compatible with the use of rutile TiO₂. In the field of photonics, such structures represent the first physically realizable self-assembled FCC (face-centered cubic) structure that possesses this functionality.

Magnetic deformation of ferrogel bodies: Procrustes effect

Deformation of spheroidal ferrogel bodies caused by a uniform magnetic field is investigated theoretically. The deformation is induced by two competitive mechanisms-magnetostatic and magnetostrictive. The former is due to the demagnetizing field of the sample and hence depends on its shape, while the latter originates from the magnetoelasticity of ferrogel and is shape independent. Both mechanisms are dipolar in nature and contribute-for a body of commensurate dimensions-oppositely to the effect. For an isotropic ferrogel sphere, the magnetostatic contribution still prevails and the magnetic field elongates the body. The two opposing mechanisms balance each other out for a prolate spheroidal sample with the axes aspect ratio a/b approximately 1.3 . It determines the so-called "Procrustes point" or "Procrustes size"-the magnetic field shrinks the body if a>1.3b and stretches it when a<1.3b .

Ultrasound attenuation in ferrofluids

The absorption of acoustic energy by internal degrees of freedom of short chains is proposed as a new viable mechanism of ultrasound attenuation in ferrofluids. It is demonstrated that even though the volume fraction of the chains may be quite small, such an effect may reach the order of magnitude of viscous damping. In addition, by investigating the statistical properties of dimers in ferrofluids, it is shown that an applied magnetic field modifies the sound attenuation in a highly anisotropic manner. The proposed mechanism provides new insight into the fundamental issue of colloidal response, and, in particular, may lead to its utilization in novel experimental concepts.

Magnetoviscosity in suspensions of grains with finite magnetic anisotropy

Coupling between magnetic and mechanical rotational degrees of freedom of fine ferromagnetic grains is provided by the energy of their magnetic anisotropy. In the limiting case of strong anisotropy, an applied stationary magnetic field induces the greatest obstacles to the "rigid dipole" spin in a vortex ferrofluid flow, while in the opposite ideal case, the "soft dipoles" twist freely with the liquid. As a result, the field-dependent part of the ferrofluids viscosity depends not only on the external magnetic field strength but also on the particle magnetic anisotropy. An explicit expression coming from simple physical arguments and describing both these dependencies of magnetoviscosity is derived and discussed.