Ashkan Davanlou

Thermally induced collision of droplets in an immiscible outer fluid

Micro-total analysis systems (μTAS) have attracted wide attention and are identified as a promising solution for sample transport, filtration, chemical reactions, separation and detection. Despite their popularity, the selection of an appropriate mechanism for droplet transport and coalescence has always been a challenge. This paper investigates the use of Marangoni flow as a mechanism for levitating and transporting droplets on immiscible liquid films at higher speeds than is possible currently. For the first time, we show that it is possible to realize the natural coalescence of droplets through Marangoni effect without any external stimulation, and deliver the coalesced droplet to a certain destination through the use of surface tension gradients. The effects of shape and size on collision outcome are studied. Regions of coalescence and stretching separation of colliding droplets are delineated based on Weber number and impact number. In addition, the effect of viscosity on post collision regimes is studied. The findings in this fundamental study can be beneficial to many applications such as welding, drug delivery and microfluidics devices in controlling small droplets and targeting them to various locations.

Droplet impact on deep liquid pools: Rayleigh jet to formation of secondary droplets.

The impact of droplets on a deep pool has applications in cleaning up oil spills, spray cooling, painting, inkjet printing, and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension, and density. Despite the exhaustive research on different aspects of droplet impact, it is not clear how liquid properties can affect the instabilities leading to Rayleigh jet breakup and number of daughter drops formed after its pinch-off. In this article, through systematic experiments we investigate the droplet impact phenomena by varying viscosity and surface tension of liquids as well as impact speeds. Further, using numerical simulations, we show that Rayleigh-Plateau instability is influenced by these parameters, and capillary time scale is the appropriate scale to normalize the breakup time. Based on Ohnesorge number (Oh) and impact Weber number (We), a regime map for no breakup, Rayleigh jet breakup, and crown splash is suggested. Interestingly, crown splash is observed to occur at all Ohnesorge numbers; however, at high Oh, a large portion of kinetic energy is dissipated, and thus the Rayleigh jet is suppressed regardless of high impact velocity. The normalized required time for the Rayleigh jet to reach its peak varies linearly with the critical height of the jet.

On the Lifetime of Non-Coalescent Levitated Droplets

It is shown that a droplet will levitate over the liquid surface for 50–700 ms when released from a critical height 1.5–4 times the droplet diameter. While releasing a droplet out of this range will lead to direct submersion. Additionally, it is shown that by applying a temperature difference between the liquid pool and droplet it is possible to elongate the levitation time of that droplet as it pulls the surrounding air molecules between the drop and the pool surface. Lastly, the thickness of the air gap is calculated theoretically for a range of temperatures and compared with experiments. Surprisingly, larger temperature difference between droplet and surface causes an increase in the thickness of the air gap. It is also found that the size of droplet and type of fluid can significantly affect the lifetime of non-coalescent drops.Copyright

Integration of Fiber Optic Sensors in Measuring Machines

The production metrology of today is still dominated by tactile probing systems. However some special metrological tasks cannot be fulfilled by this technique, one example is in the high precision manufacturing of surfaces and structures, which become ultra-miniaturized, complex and fragile. The inspection of small boreholes and cavities is also an example with very tight tolerances which demands non-contact miniaturized sensors. Particularly the measurement of the shape of spray holes in modern fuel injection nozzles for diesel engines fits this statement, as its shape represents the key factor for maximal motor efficiency, as well as minimal pollutant emissions. Any deviation from its design shape significantly affects spray breakup and can lead to unequal distribution of flow and pressure changes. These holes can have diameter of 150 microns, with a tendency to even smaller diameters in future systems.Within this work the integration of a fiber optic sensor for distance measurements in measuring machines, specifically for borehole inspection, is described. The used device is a form-tester (Mahr GmbH, MMQ-400) with 3 degrees of freedom. The motion of the machine axis will be controlled with help of image processing operation which are based on pictures taken from the specimen’s top surface. For this mean a micro camera will be mounted on the form-tester. By applying in-house developed MATLAB codes, the exact position of the boreholes and that of the fiber optic probe is obtained, so that an automated positioning and measurement (e.g. round-out and roundness tests) could be performed. This process enhances both the precision due to an optimized sensor positioning and speed of the measurement rather than manual execution. Different positioning scenarios will be discussed and compared in this paper, to prove the capability of the proposed system as well as its adaptivity.Copyright

THE EFFECTS OF SURFACTANT ON SIMPLEX NOZZLE SPRAY BEHAVIOR AND ITS COMPARISON TO LIQUID FUELS

Pressure-swirl nozzles (simplex nozzles) are used in various field applications such as aero-engines, power generation, spray painting and agricultural irrigation. For this particular nozzle, research in the past decade has dealt with the development of numerical models for predicting droplet distribution profiles. Although these results have been valuable, the experimental results have been contradictory, therefore fundamental understanding of the influence of properties in nozzle is important. This paper experimentally investigates the effect of surfactants on breakup and coalescence. Since most of the fuels and biofuels have low surface tension compared to water, a comparative analysis between a surfactant solution and a liquid fuel is imperative. For this experimental study, a simplex nozzle characterized as flow number 0.4 will be utilized. The injection pressures will range from 0.3 - 4Mpa while altering the surface tension from 72 to 28mN/m. By applying Phase Doppler Particle Anemometry (PDPA) which is a non-intrusive laser diagnostic technique, the differences in spray characteristics due to spray surface tension can be highlighted. The average droplet diameter decreases for a low surface tension fluid in the axial direction in comparison to pure water. The average velocity of droplets is surprisingly lower in the same spray zone. Measurements made in the radial direction show no significant changes, but at the locations close to the nozzle, water droplets have larger diameter and velocity. The results indicate the breakup and coalescence regimes have been altered when surface tension is lowered. A decrease in surface tension alters the breakup length while increasing the spray angle. Moreover, higher injection pressure shortens the breakup length and decrease in overall diameter of the droplets. By performing this experimental study the fundamentals of spray dynamics, such as spray formation, liquid breakup length, and droplet breakup regimes can be observed as a function of surface tension and how a surrogate fuel compares with a real fuel for experimental purposes. This knowledge potentially will lead to designing a better atomizer or new biofuels.