Biomedical subjects

Rudolf Sprik

Publications and source records attributed to Rudolf Sprik.

3 recordsLinked to original sources

PubMed2005

Wireless transfer between antenna arrays at the Talbot distance.

In modern multiple input multiple output (MIMO) wireless communication systems arrays of antennas are employed to enhance the data transfer rate. Scattering of the waves in the medium between the antennas can actually enhance the transfer capacity by decorrelating the available channels. Here it is shown that the transfer capacity between arrays of regularly spaced antennas depends intricately on the distance between the arrays and on the arrangement of scatterers. The relevant length scale is the Talbot distance L(Talbot) =2 d2/lambda known from optics as the distance where the self-imaging occurs of coherently illuminated gratings with d the grating period and lambda the wavelength. The modulation of singular values of the channel transfer matrix occurs at fractions of L(Talbot) .

Journal Article↗

PubMed2005

Spontaneously formed trans-anethol/water/alcohol emulsions: mechanism of formation and stability.

We studied the spontaneous emulsification and droplet growth mechanism in trans-anethol/water/ethanol solutions, also known as the beverage ouzo, using dynamic light scattering spectroscopy. This simple ternary mixture is a generic example of a system that forms microemulsions spontaneously when brought into the two-phase region. The volume fraction of the dispersed phase was found to profoundly affect the growth rates of the droplets, which is a new finding that has not been predicted by the Lifshitz-Slyozov-Wagner theory. Time-dependent measurements show that the droplet growth is governed by Ostwald ripening (OR), and no coalescence was observed. Furthermore, the OR rates increase with increasing oil concentration at low alcohol content. We attribute this behavior to enhanced droplet-droplet interactions. At high ethanol concentrations, we found that the measured rates decreased as the oil concentration increased. The OR growth mechanism completely correlates with changes in droplet size. The kinetics of droplet growth shows that the ripening has a saturation limit at a droplet radius of about 1.5 mum. Thus, formed emulsions remain stable for months.

Journal Article↗

PubMed2004

Time reversed wave propagation experiments in chaotic micro-structured cavities.

The elastic wave propagation in strongly scattering solid-state cavity consisting of a thin micro-patterned silicon wafer is studied experimentally. The chaotic behavior is induced by the irregular boundary of the cavity and/or by fabricating patterns of small holes in the wafer by laser machining. The pattern and hole size are designed with length scales matching the wavelength <or=1 MHz and induce multiple scattering within the wafer bounds. Regular patterns of holes add phononic band like dispersion properties to the system. Elastic waves obey under very general conditions time reversal and reciprocity symmetry. In the cavity system the strong mode mixing in the wafer between shear and compression waves, the strong anisotropy of silicon, and even small dissipation is not destroying the time reversal and reciprocity symmetry in the system. We present a systematic study of the quality of the time-reversal signal as function of the chosen time interval and the position in the recorded signal. Also the influence of time dilatation, i.e. stretching and compressing the time scale of the re-emitted signal, is studied.

Journal Article↗