The scattering of sound waves by circular-shaped clusters consisting of two-dimensional distributions of rigid cylinders in air is studied in the low-frequency limit (homogenization) both theoretically and experimentally. Analytical formulas for the effective density and sound speed are obtained in the framework of multiple scattering. Here, an experimental demonstration is reported in which a cluster of wooden rods acoustically behaves as a cylinder of argon gas. Moreover, evidence is presented indicating the validity of the homogenization in this cluster at frequencies lower than 3 kHz, which corresponds to a wavelength that is only 4 times the parameter of the embedded lattice and is a quarter of the cluster's diameter.

In this work we use multiple scattering in conjunction with a genetic algorithm to reliably determine the optimized photonic-crystal-based structure able to perform a specific optical task. The genetic algorithm operates on a population of candidate structures to produce new candidates with better performance in an iterative process. The potential of this approach is illustrated by designing a spot size converter that has a very low F-number (F=0.47) and a conversion ratio of 11:1. Also, we have designed a coupler device that introduces the light from the optical fiber into a photoniccrystal-based waveguide with a coupling efficiency over 87% for a wavelength that can be tuned to 1.5 µm.42. 42.25.Fx;42.82.Bq; A new generation of optical devices is envisaged thanks to the properties of photonic crystals (PC's).1 Though the recent advances in three-dimensional PC's structures, in the last years much attention has been focused on systems based on two-dimensional (2D) PC's because of their easiness in the fabrication process. Thus, very compact optical devices and circuits can be designed by introducing point and/or line defects. In order to use such PC circuits in actual applications it is necessary to establish a connection with an optical fiber. However, the core of the optical fiber is about one order of magnitude larger than the PC-based waveguide. Therefore, the design of an efficient (low loss) spot size converter is a crucial goal in the field of PC; its solution will introduce the PCs devices in the market place. In this regard, several groups 2-7 have tackled this problem by using different approaches. Most of them proposed tapered waveguide structures 2-5 , or by using reflective structures to focus the light into the waveguide.6 A different approach consists of using the anisotropy of the PC's equifrequency surfaces. 7This letter introduces a method that is useful in determining the optimized configuration of a 2D-PC structure capable of performing a requested optical task with high efficiency . The method is illustrated by finding a spot size converter (lens) that has a conversion ratio 11:1. In addition, the designed PC structure that involves a spot-size converter in connection with a PC-based waveguide it is presented. The insertion loss predicted for this new structure is about 13%, which is of the lowest reported by numerical simulations based on different coupling mechanisms. 2-6Our method is based on a binary-coded genetic algorithm (GA), an optimization strategy inspired by Darwinian evolution 8 . This method has been applied to solve a wide variety of problems in different fields like, for example, molecular geometry optimization 9 , material design 10 , and artificial intelligence 11 . In the field of optics, the GA has been employed in the synthesis of Bragg gratings that conform to a particular spectrum, 12 , phase recovering from a fringe pattern 13 , and in designing irregular lateral tapering. 2Although our proposal is general and applicable to any dimensionality, here we analyze 2D-PCs f...

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Emulsion formation by homogenization is commonly used in food production and research to increase product stability and to design colloidal structures. High-energy methods such as high-pressure homogenizers and rotor–stator mixers are the two most common techniques. However, to what extent does the research community understand the emulsion formation taking place in these devices? This contribution attempts to answer this question through critically reviewing the scientific literature, starting with the hydrodynamics of homogenizers and continuing by reviewing drop breakup and coalescence. It is concluded that although research in this field has been ongoing for a century and has provided a substantial amount of empirical correlations and scaling laws, the fundamental understanding is still limited, especially in the case of emulsions with a high-volume fraction of the disperse phase, as seen in many food applications. These limitations in the current understanding are also used to provide future perspectives and suggest directions for further investigation.