Solving the World’s Toughest CFD Problems

Capillary flows are commonly encountered in microfluidic devices. For example, in bio chip designs lengthy micro channels are often used to deliver liquid solution from one place to another. An inlet channel is connected to a liquid reservoir and surface tension forces pull the liquid into the micro channel (provided the liquid is “wetting” to the chip surface). This page covers some specific applications for FLOW-3D in the analysis of such capillary flows: filling, absorption and switching.

## Marangoni Flows

Marangoni flow in a dish of water that is heated at its center. Flow generated by non-uniform surface tension is demonstrated by a shallow 8.0 cm diameter dish of water of depth 0.75 cm at an initial temperature of 20º C. Placed at the center of the circular dish is a cylindrical rod of diameter 0.5 cm, which is heated to a temperature of 80 Cº and is submerged into the water surface to a depth of 0.05 cm. As the water near the hot rod is heated its surface tension is reduced by an amount of 0.1678 dyne/cm/ºC causing the surface to retract toward the outer rim of the dish. The retraction is shown by massless marker particles initially sprinkled over the surface.

## Capillary Filling

Understanding the capillary filling process is important to chip design. Different geometry of liquid flow pathways may result in different capillary filling behavior including the possibility of entrapping air bubbles, etc. Knowledge of the filling process guides designers in arranging internal structures of the chip such as chambers, binding pillars, splits, and valves. The simulation on the right validates the analytic prediction of capillary action. The capillary fill is balanced by surface tension and gravity, which is a fundamental process accurately predicted by FLOW-3D.

## Thermocapillary Switch

A small mass of liquid moved in or out of the path of a light beam can redirect it by refraction or reflection into a different path. This concept is particularly attractive in connection with optical fibers where once a beam enters a fiber it is trapped by internal reflections. To make optical circuits of any complexity, it is necessary to have a “switch” that can redirect light from one fiber to another.

One concept that has been proposed is based on thermo-capillarity. A small drop of liquid is placed in a micro channel that intersects a fiber-optic light beam. When the drop is moved along the channel to where the beam must pass through it, the beam is reflected into a different fiber. The drop is moved by differentially heating its two sides. This causes changes in the surface tension in the menisci on either side of the drop such that the drop is pulled toward the cooler end of the channel.

## Whole Blood Spontaneous Capillary Flow

Capillary-based microsystems are inexpensive and easy to fabricate because they do not need any additional actuation mechanisms. Typical microsystems like micropumps and syringes need flow actuation rendering them bulky and not portable. Recent research at University at Buffalo has investigated a simple solution for moving fluid in micro-devices using capillary flow actuation. The work uses FLOW-3D to simulate the spontaneous capillary flow in a modified V-groove channel. The narrow V-groove geometry (left) provides a good solution because high viscosity fluids like whole blood can also be moved through it. The tip of the groove facilitates the spontaneous capillary flow and the parallel plates ensure sufficient amount of blood transport.

The research uses FLOW-3D to estimate the flow velocity of the fluid head in the channel and progression of liquid front. The results are compared with experimental and analytical (simplified) results. The plots below show the comparison of numerical, experimental and analytical results. FLOW-3D results are in excellent agreement with the experimental results.

### References

J. Berthiera, K.A. Brakke, E.P. Furlani, I.H. Karampelas, V. Pohera, D. Gosselin, M. Cubizolles, P. Pouteau, Whole blood spontaneous capillary flow in narrow V-groove microchannels, Sensors and Actuators B: Chemical, 2014

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