Experimental and Numerical Characterization of Drop Impact on a Hydrophobic Cylinder

In this paper, the impact of distilled water drops on hydrophobic cylinders is characterized using both experiments and numerical simulations. Water drops of 2.54 mm in diameter impact with a velocity of 1 m/s on hydrophobic cylinders. The corresponding Reynolds and Weber numbers are 2800 and 34, respectively. Three different stainless steel cylinders with diameters of 0.48 mm, 0.88 mm, and 1.62 mm were used. The surfaces of the cylinders were made hydrophobic using a special coating spray. An experimental setup consisting of a drop generator, a high-speed camera, a lighting system, and a photoelectric sensor was used to capture images of the impact with a time-step of 1 ms. The images were then analyzed using an image processing technique implemented in the matlab software. Both the centric and off-centric impacts were studied for each cylinder diameter. A numerical simulation of the impact was also obtained using an open-source code called OpenFOAM by employing its InterFoam solver. The numerical scheme used by the solver is the volume-of-fluid (VOF) method. The predicted images of the simulations were compared well with those of the captured photographs both qualitatively and quantitatively for the entire experiments. The behavior of the drop after the impact and the subsequent deformation on hydrophobic cylinders including flow instabilities, liquid breakup, and secondary drops formation were observed from both simulations and experiments. By decreasing the cylinder diameter, the breakup occurs sooner, and a smaller number of secondary drops are formed.

MEMS-Based Microfluidic Devices

Micro-Electro-Mechanical Systems (MEMS) technology can be integrated with microfluidic functionality to enable the generation of microdrops with unprecedented throughput and precise control of drop volume, speed, and placement. The most prominent examples of microdrop generators are in the field of inkjet printing where printheads with thousands of nozzles produce steady streams of microdrops at kilohertz repetition rates. In this paper, we discuss a proposed MEMS-based microfluidic drop generator that operates on the basis of a thermally induced Marangoni effect. We describe the physics of droplet generation and discuss operating performance relative to the fluid rheology, thermal modulation, and wavelength dependencies.