Marangoni effect

[2] This phenomenon was first identified in the so-called "tears of wine" by physicist James Thomson (Lord Kelvin's brother) in 1855.

[3] The general effect is named after Italian physicist Carlo Marangoni, who studied it for his doctoral dissertation at the University of Pavia and published his results in 1865.

[4] A complete theoretical treatment of the subject was given by J. Willard Gibbs in his work On the Equilibrium of Heterogeneous Substances (1875–1878).

Water at room temperature has a surface tension of around 0.07 N/m and a viscosity of approximately 10−3 Pa⋅s.

So even variations of a few percent in the surface tension of water can generate Marangoni flows of almost 1 m/s.

For the case of a small drop of surfactant dropped onto the surface of water, Roché and coworkers[6] performed quantitative experiments and developed a simple model that was in approximate agreement with the experiments.

of a patch of the surface covered in surfactant, due to an outward Marangoni flow at a speed

with u in m/s and r in m. This gives speeds that decrease as surfactant-covered region grows, but are of order of cm/s to mm/s.

The equation is obtained by making a couple of simple approximations, the first is by equating the stress at the surface due to the concentration gradient of surfactant (which drives the Marangoni flow) with the viscous stresses (that oppose flow).

the depth into the water of the flow due to the spreading patch.

Roché and coworkers[6] assume that the momentum (which is directed radially) diffuses down into the liquid, during spreading, and so when the patch has reached a radius

the kinematic viscosity, which is the diffusion constant for momentum in a fluid.

The Marangoni number, a dimensionless value, can be used to characterize the relative effects of surface tension and viscous forces.

The effect is a consequence of the fact that alcohol has a lower surface tension and higher volatility than water.

This region with a lower concentration of alcohol (greater surface tension) pulls on the surrounding fluid more strongly than the regions with a higher alcohol concentration (lower in the glass).

The liquid will rush out of the region where the drop of alcohol fell.

Under earth conditions, the effect of gravity causing natural convection in a system with a temperature gradient along a fluid/fluid interface is usually much stronger than the Marangoni effect.

Many experiments (ESA MASER 1-3) have been conducted under microgravity conditions aboard sounding rockets to observe the Marangoni effect without the influence of gravity.

Research on heat pipes performed on the International Space Station revealed that whilst heat pipes exposed to a temperature gradient on Earth cause the inner fluid to evaporate at one end and migrate along the pipe, thus drying the hot end, in space (where the effects of gravity can be ignored) the opposite happens and the hot end of the pipe is flooded with liquid.

The fluid is drawn to the hot end of the tube by capillary action.

But the bulk of the liquid still ends up as a droplet a short distance away from the hottest part of the tube, explained by Marangoni flow.

The temperature gradients in axial and radial directions makes the fluid flow away from the hot end and the walls of the tube, towards the center axis.

The effect of the Marangoni effect on heat transfer in the presence of gas bubbles on the heating surface (e.g., in subcooled nucleate boiling) has long been ignored, but it is currently a topic of ongoing research interest because of its potential fundamental importance to the understanding of heat transfer in boiling.

One important application of the Marangoni effect is the use for drying silicon wafers after a wet processing step during the manufacture of integrated circuits.

To avoid spotting, an alcohol vapor (IPA) or other organic compound in gas, vapor, or aerosol form is blown through a nozzle over the wet wafer surface (or at the meniscus formed between the cleaning liquid and wafer as the wafer is lifted from an immersion bath), and the subsequent Marangoni effect causes a surface-tension gradient in the liquid allowing gravity to more easily pull the liquid completely off the wafer surface, effectively leaving a dry wafer surface.

Simultaneously, water condenses and forms microdroplets on the substrate.

Meanwhile, the nanoparticles in alcohol are transferred into the microdroplets and finally form numerous coffee rings on the substrate after drying.

Another application is the manipulation of particles[11] taking advantage of the relevance of the surface tension effects at small scales.

A controlled thermo-capillary convection is created by locally heating the air–water interface using an infrared laser.

The Marangoni effect is also important to the fields of welding, crystal growth and electron beam melting of metals.