In this study, the resonance of water balloons is examined, and an understanding of their dynamics in terms of our knowledge about resonating sessile drops is reported. The focus is the regime in which the competition between surface and inertial forces dominates the dynamics of the balloon. In experiments, water balloons are mechanically oscillated along their axes of symmetry, and their responses are observed through optical imaging. The observations are compared to the resonance of sessile drops. Balloons differ from drops for having elasticity in place of surface tension. Different as balloons and drops may seem, this study reveals great similarities between them: A resemblance of their normalized pressure-volume curves, a one-to-one shape correspondence between their eigenmodes, and a close approximation of their normalized resonance frequencies. Based on these, turning a drop into a balloon preserves the drop's mechanical response to a large extent. In fact, as a first approximation, the water balloons respond mechanically as water drops with a much higher surface tension.

In droplet impacts, transitions between coalescence and bouncing are determined by complex interplays of multiple mechanisms dominating at various length scales. Here we investigate the mechanisms and governing parameters comprehensively by experiments and scaling analyses, providing a unified framework for understanding and predicting the phenomena of different fluids. Specifically, while bouncing had not been observed in head-on collisions of water drops in atmospheric conditions, it was found in our experiments to appear by increasing the droplet diameter sufficiently. Contrarily, while bouncing was always observed in head-on impacts of alkane drops, we found it to disappear by decreasing the diameter sufficiently. The variations are related to gas draining dynamics in the interdroplet film and suggest an easier means for controlling bouncing as compared to alternating the ambient pressure usually labored. The scaling analysis further shows that given a Weber number, enlarging droplet diameter or fluid viscosities, or lowering surface tension contributes to a larger characteristic minimum thickness of the gas film, thus enhancing bouncing. The key dimensionless group (〖Oh〗_(g,l),〖Oh〗_l,A*) is identified, referred to the two-phase Ohnesorge number, Ohnesorge number of liquid, and the Hamaker constant, respectively. Our thickness-based model indicates that as 〖h'〗_(m,c) > 21.1h_cr, where 〖h'〗_(m,c) is the maximum value of the characteristic minimum film thickness (h_(m,c)) and h_cr the critical thickness, bouncing occurs in both head-on and off-centre collisions. That is, when (1.2〖Oh〗_(g,l))/(1-2〖Oh〗_l )>∛(A* ), a fully-developed bouncing regime occurs, thereby yielding a lowered coalescent efficiency.

The D2-Law is a commonly used relation for describing the shrinking kinetics of a single isolated droplet when it is burning. As in droplet evaporation where both conductive heat transfer and diffusive mass transfer take place simultaneously, this law is valid for droplet combustion in a microgravity environment without natural convection. Even under gravity where natural convection prevails, mounting experimental studies reveal that the shrinkage of a burning droplet still more or less obeys the D2-Law. This is, however, at odds against the dimensional consideration in that the exponent of the droplet diameter in the shrinking kinetics with gravity must depart from 2 without gravity, not to mention that physically natural convection must be at play and hence ultimately change the nature of the vaporization process.
In this talk, I will demonstrate both experimentally and theoretically that the D2-Law is indeed violated in droplet combustion under gravity. Experimentally, we determine the values of the shrinking exponent n for a variety of liquid fuels, finding n=2.25-2.45. Theoretically, we develop the first comprehensive theory for single droplet combustion by taking into account effects of natural convection with a full coupling between fluid mechanics, heat transfer, and mass transfer in the strong convection regime. Using this theory, we can uniquely determine the shrinking exponent as n=35/13~2.7 regardless of the combustion reaction kinetics. In addition, how the temperature rise, the flame dimensions, and the natural convection velocity vary with the droplet diameter can be obtained for future experimental testing. How the burning rate constant is related to fluid, thermal, and thermodynamic properties can also be determined to characterize the ability to burn for a given liquid fuel.

Pattern formation of multiphase magnetohydrodynamic (MHD) jet driven by the Lorentz force is presented. The Lorentz force generated by perpendicularly placed magnetic field and electric field displacing conductive saltwater, chemically produced gases (oxygen and hydrogen) and solid precipitates (aluminium hydroxide) by the associated reaction forms typical three-phase MHD jet flows. Taking advantage of the bright gases, the emergence of jet flow is studied by the bubbly flow. Based on the control parameters, such as magnetic field strength, input current strength, geometry of the experimental apparatus, and fluid properties, a Lorentz-force based Reynolds number Re_L is proposed to categorize the flow regime from laminar to turbulence. For jets of lower Re_L, the hydrogen/saltwater interface appears apparently more unstable, because of lighter molecular weight and more chemically produced amount, such that typical Kelvin-Helmholtz instability is observed. The distinct behaviors of oxygen and hydrogen is indistinguishable for sufficiently high Re_L due to strong dispersive mixing. Turbulent jet flow evolves once the Re_L exceeds critical value. Appropriateness of the Re_L is verified both by experiments associated with varied control parameters and corresponding numerical simulations. In addition, dependence of relevant quantitative measures of jet flows also support the transition of flow regimes determined by Re_L.

In this study, a new design concept in sweat collection was developed to achieve rapid and intact sweat sampling for analytical purposes. Textiles with fast water wicking properties were first selected and laser engraved into a tree-like bifurcating channels for sweat collection. The fractal framework of the bifurcating textile channels was theoretically derived to minimize the flow resistance for fast sweat absorption. The optimized collector with designed fractal geometry exhibited thorough coverage of emerging droplets without overflow. Great collection efficiency was achieved with a short induction time (< 1 minute after perspiration begin) and a maximum sweat collection flux up to 4.0 μL/cm2/min without leakage. After combined with printed sensors and microchips, the assembled sweat collection/sensing device can simultaneously provide measurements of salt concentration and sweat rate for wireless hydration state monitoring. The collection/sensing system also exhibited fast response times to abrupt changes in sweat rates or concentrations and thus can be used to detect instant physical conditions in exercise. Finally, field tests were performed to demonstrate the reliability and practicality of the device in real-time sweat monitoring under vigorous activities.

Satellite drops are often observed in the liquid thread pinch-off process. Whereas it is a typical Rayleigh-Plateau capillary breakup or thinning of the thread in an extensional flow, satellite drops are the universal features of the pinch-off process. The satellite drops may be desirable or undesirable depending on the type of application. For instance, satellites are undesirable in ink-jet printing while it is desirable in the spray atomization. Although it is well known that the pinch-off of the thread is always accompanied by one or more satellites, no quantitative scaling has been available to predict the size of the satellite drops. Here, we report a unique scaling for the prediction of the size of the satellite drops during a dripping process. Using experimental, numerical, and theoretical tools, we demonstrate that the size of the satellite follows a universal scaling low in the dripping regime given as dsat ~ Dw4/3, where dsat is the diameter of the satellite and Dw is the wetting diameter of the nozzle. This scaling law can be understood from the dynamics of the thread formed during the pinch-off process whose length and diameter also obey the 4/3 scaling law. A theoretical model for this scaling law is derived using a combination of Rayleigh-Plateau instability and gravitational stretching of the thread by the primary drop. The 4/3 scaling law brings a new horizon in the understanding of the size distribution of drops in a dripping process.
Keywords：Drop Formation, Satellite Drop, Drop Pinch-off

In this study, the acoustic streaming flow pattern and its mixing effectiveness in a Y-junction microchannel induced by sharp-edged triangular structures were investigated through simulations. The purpose of inducing acoustic streaming is to disturb the flow pattern around the junction region and generate a vortex stream flow, and therefore increase the vorticity to improve the performance of mixing within the entire microchannel outlet region. The Y-junction mixer is designed to be separated from each other by a 〖120〗^o gap, and includes a triangle geometry at the junction region between the two inlet sections with different widths and top tip edge angles. It also has different fillet dimensions to reduce the microchannel edge effect and increase the junction volume. The numerical model was implemented and the governing equation was solved using Multiphysics FEM. Thermoviscous acoustics, Frequency domain and Laminar Flow Physics interfaces were used to solve the acoustic field and acoustic stream flow, respectively. The effect of the induced acoustic streaming on mixing performance was also modeled. The simulation results show that acoustic streaming velocity is greatest near the triangle structure sharp top edge. It is also stronger at sharper tip edge angle. Figure 1 indicates the acoustic streaming as well as the vortex streaming in the Y-junction microchannel with 30o, 60o, and 82.37o top edge triangles geometry with 0.016µL/min flow rate and 13kHz operation frequency. The vortex streaming begins at the triangle sharp edge and rotates clockwise and anti-clockwise in each inlet direction of the junction region, as shown in figure 1. The particle and diluted species follow the steady streaming flow. Similarly, the 1"mol" ⁄"m" ^"3" fluorescein sodium salt species concentration inlets from one side but it transported easily to other side of triangle edge tip by following the streaming patterns as shown in Figure 3 when t = 200 sec. In conclusion, acoustic streaming created vortex streaming around Y-junction microchannel, and facilitating the convection transport of the molar species to the entire section of the microchannel outlet section. The effect of acoustic streaming on mixing results when the time is 200 sec, the maximum molar concentration dropped from 1 to 0.573 "mol" ⁄"m" ^"3" in the left side and the minimum concentration in the right side increased from 0 to 0.427 "mol" ⁄"m" ^"3" at y = 2.3 mm. Both concentrations of species located near to edge of its correspondence microchannel edge, which indicates that the species concentration approached to optimal 0.5 "mol" ⁄"m" ^"3" towards microchannel outlet center region.