Travis S. Emery

Circulatory shear flow alters the viability and proliferation of circulating colon cancer cells.

During cancer metastasis, circulating tumor cells constantly experience hemodynamic shear stress in the circulation. Cellular responses to shear stress including cell viability and proliferation thus play critical roles in cancer metastasis. Here, we developed a microfluidic approach to establish a circulatory microenvironment and studied circulating human colon cancer HCT116 cells in response to a variety of magnitude of shear stress and circulating time. Our results showed that cell viability decreased with the increase of circulating time, but increased with the magnitude of wall shear stress. Proliferation of cells survived from circulation could be maintained when physiologically relevant wall shear stresses were applied. High wall shear stress (60.5 dyne/cm2), however, led to decreased cell proliferation at long circulating time (1 h). We further showed that the expression levels of β-catenin and c-myc, proliferation regulators, were significantly enhanced by increasing wall shear stress. The presented study provides a new insight to the roles of circulatory shear stress in cellular responses of circulating tumor cells in a physiologically relevant model, and thus will be of interest for the study of cancer cell mechanosensing and cancer metastasis.

Bubble induced flow field modulation for pool boiling enhancement over a tubular surface

We demonstrate the efficacy of using a strategically placed enhancement feature to modify the trajectory of bubbles nucleating on a horizontal tubular surface to increase both the critical heat flux (CHF) and the heat transfer coefficient (HTC). The CHF on a plain tube is shown to be triggered by a local dryout at the bottom of the tube due to vapor agglomeration. To mitigate this effect and delay CHF, the nucleating bubble trajectory is modified by incorporating a bubble diverter placed axially at the bottom of the tube. The nucleating bubble at the base of the diverter experiences a tangential evaporation momentum force (EMF) which causes the bubble to grow sideways away from the tube and avoid localized bubble patches that are responsible for CHF initiation. High speed imaging confirmed the lateral displacement of the bubbles away from the diverter closely matched with the theoretical predictions using EMF and buoyancy forces. Since the EMF is stronger at higher heat fluxes, bubble displacement increas...

Flow Regimes and Transition Criteria during Passage of Bubbles through a Liquid–Liquid Interface

The passage of a single bubble or a stream of bubbles through a liquid-liquid interface is a highly dynamic process that can result in a number of different outcomes. Previous studies focused primarily on a single bubble and single flow regime, and very few investigations have considered bubble streams. In the present work, six different liquid combinations made up of water, ethanol, a perfluorocarbon liquid, PP1, and one of three different viscosity silicone oils are tested with air bubbles from 2 to 6 mm in diameter rising between 5 and 55 cm/s. Both single bubbles and bubble streams varying in frequency from 5 to 40 bubbles/s are tested. High-speed imaging is used to capture and classify the flow regimes associated with each flow type. Four different flow regimes are identified for single-bubble passage, and six are found for bubble stream passage. On the basis of theoretical considerations, nondimensional numbers are developed for characterizing the flow regimes and maps are generated that distinguish them and define flow regime transitions.

Interplay between developing flow length and bubble departure diameter during macroconvection enhanced pool boiling

Enhanced boiling structures based on the concept of separate liquid-vapor (L-V) pathways rely on the motion of the bubbles departing from the nucleating regions (NRs) to induce a macroconvective liquid jet impingement flow over adjacent non-boiling regions. Heat transfer in the non-boiling regions can be improved by incorporating microchannels which act as feeder channels (FCs) that also improve liquid directionality towards the NR. We hypothesize that the single-phase flow characteristics in the developing region of the FC contribute to the boiling enhancement and explore the interplay between the FC length, developing flow length, and departure bubble diameter. FC lengths shorter than the developing flow length benefit from the enhancement due to developing boundary layers over their entire length. However, FC lengths shorter than the departure bubble diameter suffer from bubble interference while FC lengths that are considerably longer than the developing flow length exhibit lower heat transfer rates in the fully developed region. This hypothesis was verified by conducting pool boiling experiments with four feeder channel lengths between 1 mm and 3 mm using HFE-7000, PP1, PP1C, and water. Three distinct regions: (i) interfering bubble, (ii) efficient L-V pathways, and (iii) diminished jet were identified to explain the boiling performance enhancement. This analysis will be beneficial in the pursuit to enhance critical heat flux (CHF) and heat transfer coefficient (HTC) on surfaces utilizing macroconvection mechanisms during boiling with different liquids.Enhanced boiling structures based on the concept of separate liquid-vapor (L-V) pathways rely on the motion of the bubbles departing from the nucleating regions (NRs) to induce a macroconvective liquid jet impingement flow over adjacent non-boiling regions. Heat transfer in the non-boiling regions can be improved by incorporating microchannels which act as feeder channels (FCs) that also improve liquid directionality towards the NR. We hypothesize that the single-phase flow characteristics in the developing region of the FC contribute to the boiling enhancement and explore the interplay between the FC length, developing flow length, and departure bubble diameter. FC lengths shorter than the developing flow length benefit from the enhancement due to developing boundary layers over their entire length. However, FC lengths shorter than the departure bubble diameter suffer from bubble interference while FC lengths that are considerably longer than the developing flow length exhibit lower heat transfer rates i...

CPU cooling with a thermosiphon loop with tapered manifold (OMM)

Data center cooling presents unique challenges to reduce global energy consumption and fluid inventory. The current work addresses these challenges by employing a thermosiphon system using two-phase cooling which improves the system efficiency through a drastic increase in heat dissipation ability. The latent heat transfer is more effective than its sensible heat counterpart. The system performance is typically characterized by its Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC). An increase in CHF offers wider operating ranges while HTC dictates the efficiency of the heat transfer process. In the current design of the cooling solution, a tapered manifold with a gap over the heater surface is employed to effectively remove the vapor away from the surface. A 3.43° tapered manifold is analyzed with HFE7000 as the working fluid in a benchtop thermosiphon. A copper chip with microchannels of width and depth of 200 μm was used for the heater surface, resulting in a CHF of 44.2 W/cm2 at a wall superheat of 16.4°C, and a maximum HTC of 27.3 kW/m2°C. This design is then arranged to be implemented as a thermosiphon Central Processing Unit (CPU) cooler. The results are compared to current cooling techniques tested on a data center server. The efficacy of the system is evaluated against its air-cooling and liquid-cooling counterparts. The thermal footprint and the system performances are evaluated for each case in this study. The cooling solution presented here has immense potential to replace existing technologies, although certain obstacles identified here will need to be considered going forth.

In vitro microfluidic circulatory system for circulating cancer cells

Circulating tumor cells (CTCs) experience hemodynamic shear stress in circulation and play critical roles in cancer metastasis. The effect of shear on CTCs, however, remains less studied. Here, we described a protocol to circulate HCT116 human colon cancer cells in a microfluidic circulatory system mimicking physiologically relevant circulating conditions. This protocol represents a useful scaffold to mimic the transportation of CTCs in circulation and thus provides an effective means to study the effect of shear on CTCs. We anticipate that future studies using the developed system will help us to further investigate the regulatory roles of shear in molecular responses of CTCs.