In this paper, we study the parameters that affect the generation of droplets in a microfluidic flow-focusing junction. Droplets are evaluated based on the size and frequency of generation. Droplet size control is essential for microfluidic lab-on-a-chip applications in biology, chemistry, and medicine. We developed a three-dimensional numerical model that can emulate the performance of the physical system. A numerical model can help design droplet-generation chips with new junction geometries, different dispersed and continuous phase types, and different flow rates. Our model uses a conservative level-set method (LSM) to track the interface between two immiscible fluids using a fixed mesh. Water was used for the dispersed phase and mineral oil for the continuous phase. The effects of the continuous-to-dispersed flow rate ratio (Qo/Qw) and the surfactant concentration on the droplet generation were studied both using the numerical model and experimentally. The numerical model was found to render results that are in good agreement with the experimental ones, which validates the LSM model. The validated numerical model was used to study the time effect of changing Qo/Qw on the generated droplet size. Properly timing when the flow rates are changed enables control over the size of the next generated droplet, which is useful for single-droplet size modulation applications.

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In this paper we study the dynamics of single cells encapsulated in water-in-oil emulsions in a microchannel. The flow field of a microfluidic channel is coupled to the internal flow field of a droplet through viscous traction at the interface, resulting in a rotational flow field inside the droplet. An encapsulated single cell being subjected to this flow field responds by undergoing multiple orbits, spins, and deformations that depend on its physical properties. Monitoring the cell dynamics, using a high-speed camera, can lead to the development of new label-free methods for the detection of rare cells, based on their biomechanical properties. A sheath flow microchannel was proposed to strengthen the rotational flow field inside droplets flowing in Poiseuille flow conditions. A numerical model was developed to investigate the effect of various parameters on the rotational flow field inside a droplet. The multi-phase flow model required the tracking of the fluid–fluid interface, which deforms over time due to the applied shear stresses. Experiments confirmed the significant effect of the sheath flow rate on the cell dynamics, where the speed of cell orbiting was doubled. Doubling the cell speed can double the amount of extracted biomechanical information from the encapsulated cell, while it remains within the field of view of the camera used.

In the current study, the effect of using heat storage materials aimed at the night shift operation was studied by simulation tools. The Finite Volume Method (FVM) is used as a method to discretize the governing equations in the 2-D domain, and the RNG K- turbulence model is applied for simulation. The effect of absorption type based on five different materials such as Aluminum, Water, Asphalt, Graphite, and Sand on the performance of solar air heaters (SAH) is carried out thermally. The results show that the optimum material to obtain the maximum difference in temperature (38.32 K) between the absorber and the surrounding occurs once the asphalt is used, while the sand is the worst case, which is (31.71 K). Following, the focus is on the best absorber for night time operation. The Lumped Parameter Analysis is applied to the time required to dissipate the heat energy from the storage layer to reduce it to the assumed final temperature of a 5-degree difference (higher) than the ambient. After solving the transient equation, it is found that water can operate the system for a longer period (4.31 hrs.) as well as Graphite can operate the system for a shorter period (0.46 hrs.). Then ten different velocities in the entry section are used to consider the influences on the system performance. The results show that the velocity has a significant effect on the outlet temperature and operation period for night time.