Petr Denissenko

Human spermatozoa migration in microchannels reveals boundary-following navigation

The migratory abilities of motile human spermatozoa in vivo are essential for natural fertility, but it remains a mystery what properties distinguish the tens of cells which find an egg from the millions of cells ejaculated. To reach the site of fertilization, sperm must traverse narrow and convoluted channels, filled with viscous fluids. To elucidate individual and group behaviors that may occur in the complex three-dimensional female tract environment, we examine the behavior of migrating sperm in assorted microchannel geometries. Cells rarely swim in the central part of the channel cross-section, instead traveling along the intersection of the channel walls (“channel corners”). When the channel turns sharply, cells leave the corner, continuing ahead until hitting the opposite wall of the channel, with a distribution of departure angles, the latter being modulated by fluid viscosity. If the channel bend is smooth, cells depart from the inner wall when the curvature radius is less than a threshold value close to 150 μm. Specific wall shapes are able to preferentially direct motile cells. As a consequence of swimming along the corners, the domain occupied by cells becomes essentially one-dimensional, leading to frequent collisions, and needs to be accounted for when modeling the behavior of populations of migratory cells and considering how sperm populate and navigate the female tract. The combined effect of viscosity and three-dimensional architecture should be accounted for in future in vitro studies of sperm chemoattraction.

An experimental study on hurricane mesovortices

Mesovortices in the eyewall region of a hurricane are intriguing elements of the hurricane engine. In-situ measurements of them are sparse, however, and our understanding of their overall role in the physics of a hurricane is incomplete. To further understand their dynamics an experimental apparatus using a homogeneous fluid (water) has been constructed to emulate the lower tropospheric flow of the hurricane eye/eyewall region. For experimental configurations possessing a central aspect ratio less than unity, a primary and secondary circulation similar to the in flow layer of an intense hurricane, and a similar radius-to-width ratio of the curvilinear shear layer bordering the eye and eyewall region, the flow supports two primary quasi-steady vortices and secondary intermittent vortices. The vortices form through Kelvin–Helmholtz instability of the curvilinear shear layer bordering the slowly upwelling fluid in the centre and the converging fluid from the periphery. The primary vortices are maintained by convergence of circulation from the periphery and merger of secondary vortices spawned along the shear layer. The horizontal flow field is measured using a particle image velocimeter. Despite the relatively strong secondary circulation through the parent vortex the horizontal flow is found to be approximately uniform in the direction parallel to the rotation axis. The peak tangential velocity is found to occur in the mesovortices and is roughly 50% greater than the parent vortex that supports them. The measurements provide insight into recent observations of excessive wind damage in landfalling storms and support the hypothesis that intense storms contain coherent vortex structures in the eyewall region with higher horizontal wind speeds locally than the parent hurricane.

Statistics of surface gravity wave turbulence in the space and time domains

We present experimental results on simultaneous space-time measurements for the gravity wave turbulence in a large laboratory flume. We compare these results with predictions of the weak turbulence theory (WTT) based on random waves, as well as with predictions based on the coherent singular wave crests. We see that both wavenumber and the frequency spectra are not universal and dependent on the wave strength, with some evidence in favor of WTT at larger wave intensities when the finite flume effects are minimal. We present further theoretical analysis of the role of the random and coherent waves in the wave probability density function (PDF) and the structure functions (SFs). Analyzing our experimental data we found that the random waves and the coherent structures/breaks coexist: the former show themselves in a quasi-gaussian PDF core and in the low-order SFs, and the latter - in the PDF tails and the high-order SFs. It appears that the x-space signal is more intermittent than the t-space signal, and the x-space SFs capture more singular coherent structures than do the t-space SFs. We outline an approach treating the interactions of these random and coherent components as a turbulence cycle characterized by the turbulence fluxes in both the wavenumber and the amplitude spaces.

The flow generated by an active olfactory system of the red swamp crayfish

SUMMARY Crayfish are nocturnal animals that mainly rely on their chemoreceptors to locate food. On a crayfish scale, chemical stimuli received from a distant source are dispersed by an ambient flow rather than molecular diffusion. When the flow is weak or absent, food searching can be facilitated by currents generated by the animal itself. Crayfish employ their anterior fan organs to produce a variety of flow patterns. Here we study the flow generated by Procambarus clarkii in response to odour stimulation. We found that while searching for food the crayfish generates one or two outward jets. These jets induce an inflow that draws odour to the crayfishs anterior chemoreceptors. We quantified velocity fields in the inflow region using Particle Image Velocimetry. The results show that the inflow velocity decreases proportionally to the inverse distance from the animal so that it takes about 100 s for an odour plume to reach the animals chemoreceptors from a distance of 10 cm. We compare the inflow generated by live crayfish with that produced by a mechanical model. The model consists of two nozzles and an inlet and provides two jets and a sink so that the overall mass flux is zero. Use of the model enabled us to analyze the inflow at various jet parameters. We show that variation of directions and relative intensities of the jets allows the direction of odour attraction to be changed. These results provide a rationale for biomimetic robot design. We discuss sensitivity and efficiency of such a robot.

Algal motility measured by a laser-based tracking method

The velocities of individual actively swimming cells of the green alga Chlamydomonas nivalis can be measured with the use of spatial displacements of their successive images (by a ‘tracking’ method). This paper proposes a new tracking method based on the use of laser light; it is common for the tracking of passive particles, but the case of actively swimming cells presents specific problems. The first problem is the potential influence of the laser light on the behaviour of the cells. A key result of the study is that for carefully chosen parameters the effect of the laser light on the algal swimming is negligible (below the measurement error). Hence, the self-swimming velocities can be measured; sample results are given. The proposed method permits a large measurement volume, and thus allows the velocities of several hundreds of cells to be measured simultaneously. From these data an approximation for the probability distribution function of the velocities of the cells can be derived; this function is crucial for the construction of mathematical models of algal motility. The laser-based tracking method is applicable and useful for measurements of algal motion in still fluid. The method thus presents new opportunities in this area of research.

Effect of asymmetry of incident wave on the maximum runup height

ABSTRACT Didenkulova, I., Denissenko, P., Rodin, A. and Pelinovsky, E., 2013. Effect of asymmetry of incident wave on the maximum runup height. Shoaling and runup of long waves on a beach form a classical task for coastal oceanography and engineering. Though many empirical and theoretical formulae have been developed in this field, most of them are targeted to typical waves and situations, while the greatest hazard is caused by extreme events, such as, for example, extreme storms and catastrophic tsunamis. From this point of view it was shown theoretically that one of the most important parameters which influence the wave runup height is the steepness of the incident wave front and the asymmetry of the incident wave. It helped to provide a simple explanation to the extreme runup observed during the catastrophic 2004 Indonesian tsunami event. However, the theoretical results were obtained under many assumptions (ideal fluid, no wave breaking, no bottom friction) and have not been validated. Here we present an experimental study performed in the Large Wave Flume (GWK), Hannover, Germany, which is focused on the influence of the asymmetry caused by the non-linear deformation of incident waves on their runup on a plane beach. The series of experiments are aimed to validate the theoretical formulae for runup height of asymmetric waves. Obtained results are in a good agreement with theoretical predictions and corresponding formulae are recommended to be considered in wave forecasts.

Experimental statistics of long wave runup on a plane beach

ABSTRACT Denissenko, P., Didenkulova, I., Rodin, A., Listak, M. and Pelinovsky, E., 2013. Experimental statistics of long wave runup on a plane beach. A series of experiments at the 300 m length and 3.5 m depth Large Wave Flume (GWK), Hannover, Germany is conducted to study the statistics of long wave runup on a plane beach. Major goal of this experimental study is to test the applicability of the theoretical model based on rigorous solutions of the nonlinear shallow-water equations for waves on a sloping beach. According to the theory, the extreme values of the runup displacement and velocity of the moving shoreline, and consequently statistical distributions of extreme runup characteristics can be found from the associated linear theory. In particular, if an incident wave field is represented by a narrow-band Gaussian process, extreme runup characteristics can be described by the Rayleigh distribution even for strongly nonlinear waves. It is shown that the distribution function of the extreme runup characteristics can be approximated by the Rayleigh curve in the wide range of wave amplitudes and spectra even if an incident wave field is represented by a non-Gaussian process.

Experimental molecular communications in obstacle rich fluids

A key potential advantage of molecular communications is the ability of molecules to propagate in complex propagation channels. Here, we experimentally test the information rate in both relatively laminar and turbulent conditions by tracking the information molecules using particle image velocimetry (PIV). A number of obstacle types are placed in the channel and we observe that they do not generally lower the information rate, but may actually improve it in some cases. This is explained by the formation of self-sustaining coherent vortex signal structures with a higher signal-to-noise ratio (SNR), which are caused by obstacles. The initial results demonstrate experimentally that the variety of obstacles tested do not impact data rate and may in some cases enhance it.

Surfactant Concentration And Its Effect On The Flow Field Inside An Oil Droplet Rising In An Aqueous Medium: LIF And PIV Measurements

We explore the effects of adsorption of a fluorescent surfactant-like substance (Rhodamine-6G) to the interface of a droplet of mineral oil rising through a waterglycerol mixture with Re 1. As Rhodamine-6G adsorbs to the interface it collects in a stagnant cap at the base of the droplet. This in turn impedes the internal flow of the droplet. Laser-Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) are used as measurement techniques to explore the effects of Rhodamine-6G adsorbing to the oil-aqueous interface. LIF is used to observe the transport of surfactant (Rhodamine-6G) as it adsorbs to the front facing section of the droplet interface and desorbs in the wake behind the droplet. The LIF visualisation shows the accumulation of Rhodamine-6G at the rear of the droplet in the form of a cap, as well as the formation of a surfactantdepleted tail behind the droplet. This cap grows in size as the droplet rises and develops at different rates for different concentrations of Rhodamine-6G. PIV measurements are obtained to visualise the circulation inside the droplet and to measure the angle of the stagnant cap as the droplet rises, with these measurements being linked to the LIF results.

Positron emission tracking of individual particles in particle-laden rimming flow

The motion of a single tracer particle in particle-laden rimming flows is investigated experimentally by means of Positron Emission Particle Tracking (PEPT). Semi-dilute suspensions, with a volume fraction of 8% of heavy particles are considered. The trajectory of the tracer particle is monitored for several thousand cylinder revolutions and related to the optically recorded drift of the large-scale granular segregation bands developing in the cylinder. Results of the data analysis provide first insights into the relation between behaviour of individual particles and the spatiotemporal dynamics displayed by the macroscopic particle-segregation patterns.