We report on the behavior of small particles of dilute concentration in time-dependent (oscillatory) thermocapillary flow in cylindrical liquid bridges. The particles accumulate in a dynamic string for certain aspect ratios of the liquid bridge and at, typically, two times the critical Marangoni number for the onset of time dependence. This was observed for particles with a density larger and smaller than that of the fluid and for the isodense case. If looked at in a snapshot, this string would be wound $m$ times around the thermocapillary vortex as a deformed spiral. If one looked at the full dynamics, it would be seen that the spiral string is rotating around its ring-shaped axis. The phenomenon is called a dynamical particle accumulation structure (dynamical PAS). The mode $m$ is the mode number of the oscillatory flow field with $m$ wavetrains of the hydrothermal wave (HTW) traveling in the azimuthal direction. We visualize and describe the different modes $m$ in detail. We give direct experimental evidence for the gathering of liquid with particles during the cold phases of the HTW and the injection of liquid with particles into the return flow in azimuthally traveling “cold spots.” We varied the particle diameter at constant density and the ratio of the particle density to fluid density at constant particle diameter to measure the time of the formation of PAS and discuss and explain the experimental results in comparison with possible mechanisms underlying the formation process. We describe the results of an experiment under microgravity to exclude gravity as a PAS-forming mechanism. We conclude by describing a possible mechanism that could account for the observed particle accumulation in certain regions of the flow. This mechanism involves the observed gathering and injection of liquid during the cold phases of the HTW and the particle enrichment of the injected fluid due to particle migration in sheared flow. PAS occurs at a resonance between the azimuthally traveling wave and the “turnover time” of the PAS-string in the thermocapillary vortex.

We present the results of an experimental study of convective instabilities in a horizontal liquid layer with free upper surface under the action of an inclined temperature gradient, i.e., when horizontal and vertical temperature gradients are applied at the same time. Silicone oil of 10 cSt (Prandtl number $\text{Pr}=102$ ) was employed as the test fluid. We investigated the layers with different thicknesses to examine the influence of gravity on the formation of the convective patterns. It is found out that the system behavior appreciably depends on the dynamic Bond number, which shows a relation of buoyancy and thermocapillary forces. In the case of small dynamic Bond numbers, when the influence of buoyancy is minimal, four different flow patterns, according to the combination of the vertical and horizontal Marangoni numbers, have been found: steady parallel flow, Bénard–Marangoni cells, drifting Bénard–Marangoni cells, and longitudinal rolls. At larger dynamic Bond number, when the influence of buoyancy becomes considerable, new convective structures, named by us the “surface longitudinal rolls” and the “surface drifting cells,” appear in addition to the patterns listed above. These instabilities exist only in the surface part of the thermocapillary flow, whereas the return flow remains stable. Under large enough dynamic Bond number these patterns become the dominating ones, forcing out the classical Bénard–Marangoni instability. We give a phenomenological description of the obtained convective patterns and present the stability diagram in the plane of the vertical and the horizontal Marangoni numbers.

Timely diagnostics of microcirculatory system abnormalities, which are the most severe diabetic complications, is one of the major problems facing modern health care. Functional abnormalities manifest themselves earlier than the structural ones, and therefore their assessment is the issue of primary importance. In this study Laser Doppler flowmetry, a noninvasive technique for the cutaneous blood flow monitoring, was utilized together with local temperature tests and wavelet analysis. The study of the blood flow in the microvascular bed of toes was carried out in the control group of 40 healthy subjects and in two groups of 17 type 1 and 23 type 2 diabetic patients. The local temperature tests demonstrated that the diabetic patients have impaired vasodilation in response to local heating. The tendency for impaired low frequency pulsations of the blood flow associated with endothelial and neurogenic activities in both diabetes groups was observed. Local thermal tests induced variations in perfusion and its spectral characteristics, which were different in the groups under study. In our opinion, the obtained preliminary results can be a basis for further research and provide a deeper understanding of pathological processes that drive microvascular abnormalities caused by diabetes mellitus.

Photoplethysmography (PPG) and laser Doppler flowmetry (LDF) are two recognized optical techniques that can track low-frequency perfusion changes in microcirculation. The aim of this study was to determine, in healthy subjects, the correlation between the techniques for specific low-frequency bands previously defined for microcirculation. Twelve healthy male subjects (age range 18 to 50 years) were studied, with PPG and LDF signals recorded for 20 min from their right and left index (PPG) and middle (LDF) fingers. Wavelet analysis comprised dividing the low-frequency integral wavelet spectrum (IWS) into five established physiological bands relating to cardiac, respiratory, myogenic, neurogenic, and endothelial activities. The correlation between PPG and LDF was quantified using wavelet correlation analysis and Spearman correlation analysis of the median IWS amplitude. The median wavelet correlation between signals (right-left side average) was 0.45 (cardiac), 0.49 (respiratory), 0.86 (myogenic), 0.91 (neurogenic), and 0.91 (endothelial). The correlation of IWS amplitude values (right-left side average) was statistically significant for the cardiac ($ρ=0.64$, $p<0.05$) and endothelial ($ρ=0.62$, $p<0.05$) bands. This pilot study has shown good correlation between PPG and LDF for specific physiological frequency bands. In particular, the results suggest that PPG has the potential to be a low-cost replacement for LDF for endothelial activity assessments.

We report on chemoconvective pattern formation phenomena observed in a two-layer system of miscible fluids filling a vertical Hele-Shaw cell. We show both experimentally and theoretically that the concentration-dependent diffusion coupled with frontal acid-base neutralization can give rise to the formation of a local unstable zone low in density, resulting in a perfectly regular cell-type convective pattern. The described effect gives an example of yet another powerful mechanism which allows the reaction-diffusion processes to govern the flow of reacting fluids under gravity conditions.

The structure and stability of thermocapillary and solutocapillary flow from a localized source has been experimentally studied in the presence of adsorption layers. It is found that the divergent flow, which is typical for this case, becomes unstable to azimuthally periodic perturbations that lead to the appearance of a surface flow with a multivortex structure. The evolution of the flow structure during an increase of concentration in the adsorption layer is shown. Two possible physical models of the observed instability are put forward.