Alem Teklu

Experimental study of nonequilibrium fluctuations during free diffusion in nanocolloids using microscopic techniques

We report quantitative experimental results regarding concentration fluctuations based on a small-angle light-scattering setup. A shadowgraph technique was used to record concentration fluctuations in a free-diffusion cell filled with colloids. Our experimental setup includes an objective attached to the CCD camera to increase the field of view. We performed two separate experiments, one with 20 nm gold and the other with 200 nm silica colloids, and extracted both the structure factors and the correlation time during the early stages of concentration fluctuations. The temporal evolution of fluctuations was also qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. We found that the correlation time versus wavenumber for gold nanocolloids is concave shaped, whereas, for silica colloids, it is convex shaped. The difference in correlation time behavior is not only due to the size of the particle, but also to possible plasmonic interactions in gold colloids.

Measurement of the frequency dependence of the ultrasonic parametric threshold amplitude for a fluid-filled cavity

By driving a transducer at one end of a fluid-filled cavity parallel to a rigid plane reflector at the other end, standing ultrasonic waves can be generated. Variations in the cavity length resulting from transducer motion lead to the generation of resonant frequencies lower than the drive frequency (known as fractional harmonics). This excitation of fractional harmonics in a liquid-filled cavity by ultrasonic waves was described previously as a parametric phenomenon [Laszlo Adler and M. A. Breazeale, J. Acoust. Soc. Am. 48, 1077–1083 (1970)]. This system was modeled by using a modified Mathieu’s equation whose solution resulted in the prediction of critical threshold drive amplitude for the excitation of parametric oscillation. The apparatus used by Adler and Breazeale was recently refined for accurate measurements of the threshold amplitude for parametric excitation at frequencies ranging from 2 to 7MHz. The measurements showed that in this range the threshold amplitude increases with increasing drive f...

Optical Bragg imaging of acoustic fields after reflection.

Bragg diffraction of x-rays occurs when the rays interact with a crystalline lattice at the appropriate angle. Bragg diffraction of visible light occurs when the light interacts at the Bragg angle with an ultrasonic field of the appropriate frequency. (The spacing between acoustic condensations and rarefactions acts like the planes in an atomic lattice.) If a beam of light is Bragg diffracted by an ultrasonic beam that previously has passed through an object, an image of the structure of the object is made visible in the diffraction field of the optical beam since there is a one-to-one mapping of the ultrasonic field onto the diffraction order. In many acoustic Bragg imaging applications, the sound field must pass through the object which is to be imaged. Ultrasonic attenuation at the very high acoustic frequencies needed for Bragg imaging (typically approximately 25-30 MHz) severely limits the nondestructive testing (NDT) applications of traditional acoustic Bragg imaging. In this paper, a reflection-based application of acoustic Bragg imaging is discussed which may have useful industrial and biomedical NDT applications.

Cost-effective potential application of acousto-optic Bragg imaging of biological tissue

Acousto-Optic Bragg Imaging is a technique that uses the interaction of light with ultrasound to optically image obstructions in acoustical fields. Existing reports of Acousto-Optic Bragg Imaging based on transmission of acoustic fields through obstructions exhibit strong acoustic impedance mismatches manifested by poor image quality and missing details of physical structures of obstructions. In this work, the image quality was improved to exhibit detailed physical structures of an object by using an improved Bragg imaging system. This project investigates the possibility of extending an acoustic Bragg imaging technique in transmission mode to image animal or plant tissues; a small Azalea leaf is used as an illustration in this case. The Bragg image produced clearly shows the veins of the vascular azalea leaf serving as a proof of concept for cost-effective potential application of acoustic Bragg imaging of biological objects in the medical field. Moreover, acousto-optic Bragg imaging is potentially harml...

Acousto-optic Bragg imaging of biological tissue

Acousto-optic Bragg imaging is a technique that uses the interaction of light with ultrasound to optically image obstructions in acoustical fields. Existing reports of acousto-optic Bragg imaging based on transmission of acoustic fields through obstructions exhibit strong acoustic impedance mismatches manifested by poor image quality and missing details of physical structures of obstructions. In this work, the image quality was improved to exhibit detailed physical structures of an object by using an improved Bragg imaging system described in Sec. III below. This paper investigates the possibility of extending an acoustic Bragg imaging technique in transmission mode to image animal or plant tissues; a small azalea leaf is used as an illustration in this case. The Bragg image produced clearly shows the veins of the vascular azalea leaf serving as a proof of concept for cost-effective potential application of acoustic Bragg imaging of biological objects in the medical field. Moreover, acousto-optic Bragg imaging is potentially harmless to biological cells and is sensitive to density and elastic variations in the tissue.

Frequency dependence of the ultrasonic parametric threshold amplitude for a fluid‐filled cavity.

The excitation of fractional harmonics in a liquid‐filled cavity by ultrasonic waves was described previously as a parametric phenomenon [L. Adler and M. A. Breazeale, J. Acoust. Soc. Am. 48, 1077–1083 (1970)]. That is, by driving a transducer at one end of a fluid‐filled cavity parallel to a rigid plane reflector at the other end, standing ultrasonic waves can be generated. Variations in the cavity length resulting from transducer motion lead to the generation of resonant frequencies lower than the drive frequency (known as fractional harmonics). The system was modeled by using a modified Mathieus equation whose solution resulted in the prediction of critical threshold drive amplitude for the excitation of parametric oscillation. The apparatus used by Adler and Breazeale was recently refined for accurate measurements of the threshold amplitude for parametric excitation at frequencies ranging from 2 to 7 MHz. The measurements showed that in this range the threshold amplitude increases with increasing driv...

Fiber direction determination in composites by means of Schlieren photography

After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, the paper describes and experimental method, based on the combination of Schlieren photography and a wide bounded ultrasonic beam of frequency 10 MHz, used to detect the fiber direction in composites. Experiments are described on unidirectional and fabric fiber reinforced composites. This method is not based on Lamb wave phenomena, which can be complicated for interpretation by NDT engineers, but rather is based on relatively simple principles, i.e. inhomogeneity and symmetry of composites.

Schlieren photography as a great tool to study wave diffraction at the end of a plate

A study by means of the Schlieren technique for visualization of ultrasonic beams, has revealed that when leaky Rayleigh waves, propagating along the horizontal edge of a thick, fluid loaded solid plate, are scattered at the extremity of the plate, they travel around the corner and start leaking into the liquid along the Rayleigh angle measured from the normal to the vertical edge of the plate. Furthermore the study reveals that leaky Rayleigh waves are stimulated by the border of an incident ultrasonic bounded beam, more than by the interior of the beam. Comparison with an earlier work shows that the characteristics of the scattering of leaky Rayleigh waves at the edge of the plate is very different from that of Scholte - Stoneley waves.

The Schoch effect as a means to detect the principal axes of a piezoelectric crystal

When a bounded beam is incident on a piezoelectric crystal at the right angle, surface waves are generated which result in the Schoch effect. This effect consists of the appearance of two reflected beams instead of one, with a null strip in between. The characteristics of this Schoch effect are determined by the physical properties of the solid under consideration, but also by the direction of the principal axes. Therefore, the effect can be used to determine those directions. A comparison of theory and experiments on lithium niobate is shown. This method can be used as a practical tool for determining the principle axes, since these are not visible to the eye. [Work supported by The Flemish Institute for the Encouragement of the Scientific and Technological Research in Industry (I.W.T.) and by a NATO Collaborative Linkage Grant.]

The diffraction of leaky Rayleigh waves at the extremity of a fluid‐loaded plate

This study reveals that leaky Rayleigh waves, when scattered at the extremity of a thick plate swamped in water, generate leaky Rayleigh waves that propagate around the corner. Furthermore, it is experimentally proved that leaky Rayleigh waves are stimulated by the borders of a Gaussian‐bounded beam and not by the interior of the beam. A comparison between the scattering of leaky Rayleigh waves and the scattering of Scholte–Stoneley waves at the extremity is also outlined and shows that leaky Rayleigh waves on the vertical edge at the extremity of the plate are best stimulated by means of incident leaky Rayleigh waves and not by means of Scholte–Stoneley waves. [Work supported by The Flemish Institute for the Encouragement of the Scientific and Technological Research in Industry (I.W.T.) and by a NATO Collaborative Linkage Grant.]