We numerically investigate the effects of bulk fluid viscoelasticity on droplet formation and dynamics in an axisymmetric flow focusing configuration. Viscoelasticity is modeled using the finitely extensible nonlinear elastic-Chilcott-Rallison (FENE-CR) model. Extensive simulations are performed to examine droplet formation and breakup dynamics for a wide range of parameters including flow rate ratio, Weissenberg number, polymeric viscosity ratio, and extensibility parameter. It is found that these parameters have a significant influence on the droplet size and size distribution (dispersity). Three different regimes are observed in the sequence of squeezing, dripping, and jetting modes as the flow rate ratio is increased. It is also found that the viscoelasticity has a similar effect as decreasing flow rate ratio and acts to delay transition from squeezing to dripping and from dripping to jetting regimes. The strain-rate hardening occurs at a critical Weissenberg number resulting in an abrupt increase in droplet size and this effect is more pronounced as the polymeric viscosity ratio is increased.

Analytical prediction of electrowetting-induced jumping motion for droplets on hydrophobic substrates

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Encapsulated cell therapy has shown great potential in the treatment of several forms of cancer. Microencapsulation of these cancer cells can protect the core from the harmful effects of the neighboring cellular environment and can supply nutrients and oxygen. Such an encapsulation technique ensures cell viability and enables targeted drug delivery in cancer therapy. The cells immobilized with a biocompatible shell material can be isolated from the ambient and can move in constricted microcapillary. However, transportation of these cells through the narrow microcapillary may squeeze and mechanically damage the cells which threaten the cell viability. The cell type, conditions and the viscoelastic properties of the shell can dictate cell viability. A front-tracking numerical simulation shows that the engineered shell material with higher viscoelasticity improves the cell viability. It is also shown that low cortical tension of cells can contribute to lower cell viability. Cancer cell mechanics have been extensively studied in microfluidic systems for cell sorting 1 , cell banking 2 and cancer therapy 3. Several research papers have classified cancer cells based on their mechanical properties, which can be utilized for the development of innovative diagnostic devices 3-6. The cancer cell deformation has been studied as a tool to sort healthy and unhealthy cells for cancer therapies and to disrupt the metastatic process 3. Microfluidic optical stretchers can deliver individual cells 7. The experiments on malignant transformation of human breast epithelial cells characterized the relationship between cellular function and cytoskeletal mechanical properties. The cancer cells may deform up to five times more than the healthy cells and that the metastatic cancer cells have a tendency to deform more than the non-metastatic cancer cells 7. Encapsulation of various cancer cells has been done over long culture periods using tumor microsphere and spheroids models 2. This study showed a higher uniformity and lower variability in diameter and circularity of the tumor microspheres over self-aggregated tumor spheroids, however both models can assure high cell viability. A mathematical model also has been developed to characterize the cellular interaction with endothelium that showed a larger deformation of cancer cells compared to healthy cells during metastasis 5. Thus, while encapsulation techniques in microchannels improve cell viability and target delivery, it is important to study the deformation of encapsulated leukemia cells in constricted microchannels. Although numerous experiments have been conducted in cell biophysics, modeling simulations of cell mechanics in bio-microfluidic systems have been lacking largely due to the complexity of the system. Lykov et al. 8 reviewed recent modeling approaches for cell mechanics simulations. It is possible to develop computational models to study the flow phenomena for different suspended cells such as white blood cells (WBC), circulating tumor cells (CTC) and other cancer cells. T...

Cell microencapsulation is a promising technique to protect living cells in biomedical applications. Microfluidic devices can be utilized to control the production of high-throughput cell-laden droplets. This paper demonstrates the effects of flow-focusing geometry on the droplet size, frequency of droplet generation, and number of cells per droplet. Orifice radius, orifice length, and nozzle-to-orifice distance can significantly influence the flow-field and manipulate droplet formation. This paper analyzes these geometry effects using a numerical front-tracking method for the three fluid phases. It is found that as the orifice radius increases, the drop size and the number of cells in the droplet increase. For a short orifice radius, increasing the orifice length results in the generation of smaller droplets at higher frequency and fewer cells per droplet. On the other hand, for a longer orifice, droplet production is invariant with respect to orifice length. It is also found that shorter distances between the nozzle and the orifice lead to a more controlled and uniform production of droplets. When the nozzle-to-orifice length is increased, the droplet formation becomes non-uniform and unpredictable. Probability charts are plotted with respect to the orifice length and orifice radius, which show that a greater than 50 % probability of single cell encapsulation can be achieved consistently.