Effect of lubrication forces on fluid collisions

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2013

New research determines quantitatively how fluid mixing can lead from noncoalescence to coalescence.

KEY CONCEPTS
• The assumption that two fluid streams always mix together to form one fluid was refuted in a 19th Century observation that two fluids can bounce off each other in a process called noncoalescence.
• New research using two liquid silicon oil jets shows that velocity is the key factor in determining whether the fluids coalesce or bounce off each other.
• By increasing their speed, the fluids move through a transition phase from bouncing to coalescence that may occur as the air film between the jets becomes thinner.

AT SOME POINT IN TIME we have all mixed two fluids together, whether in preparing a lubricant blend with basestocks of different viscosities or even mixing blue with red paint in an effort to get the right shade of purple. In a production facility, a variety of mixing equipment is available ranging from stirring bars in the laboratory to mixing vessels containing various types of agitators.

In a previous TLT article, two new techniques for mixing were described that utilize magnetism (1). Introduction of four-to seven-micron-diameter iron or nickel particles in a fluid leads to their dispersion as countless microscopic stir bars in a method known as vortex field mixing. This approach can become cumbersome, particularly at large production scales, so a second method known as isothermal magnetic advection was developed that suspends thin, 20-micron magnetic platelets in a fluid, which are subjected to a biaxial magnetic field creating the mixing effect.

The assumption can be made that mixing two fluids together should always lead to their merger into one fluid. But this is not always the case. Sunghwan Jung, assistant professor of engineering science and mechanics at Virginia Tech in Blacksburg, Va., says, “It is possible for two fluids not to merge into one fluid upon mixing. Our expectation from evaluating the properties of small liquid drops is that merger should always take place so that two small drops become a larger third drop. But Lord Baron Rayleigh, an English physicist, discovered in the 19th Century that two fluid jets do not always merge together into one uniform liquid. Instead the two fluid jets can bounce off each other.”

This process is known as noncoalescence, and this effect can have an impact on a variety of applications such as optimizing fuel efficiency in a rocket where two fluids need to mix well to produce the desired combustion effect. Other applications include inkjet printing and the development of spray coatings.

Noncoalescence occurs due to the presence of a lubrication film in an air film that forms between the two fluids. Jung explains, “As the two liquid jets move together, they both carry air, which forms a film between them because of the presence of lubrication forces. When the air film is squeezed, this force resists the merger of the two liquid jets.”

A better understanding of fluid mixing may enable improvements to be made in these applications. New work is being done to determine quantitatively how fluid mixing can lead from noncoalescence to coalescence.

JET VELOCITY
Jung, in collaboration with Pavlos Vlachos, professor of mechanical engineering at Virginia Tech, and graduate student, Navish Wadhwa, devised a series of experiments to take a closer look at the interactions of two liquid jets. A series of silicone oils with viscosities ranging from 4.4 to 46.6 cSt at 25 C were studied. Jung says, “We decided to work with silicone oils because these fluids exhibit clean, uniform surfaces due to their low surface tensions.”

Solutions of 75 percent glycerol in water were also evaluated in a few experiments. These fluids exhibited a similar range of viscosity as the silicone oils but display a higher surface tension that was 66 millinewton per meter. Jung says, “We used the glycerol solution to see how a system with a higher surface tension compared to silicone oil. It would have been ideal to use additives such as surfactants to reduce the surface tension, but that is not practical in this type of experiment.”

The researchers set up two blunt tip, stainless steel needles with a circular cross section so that the fluid jets appeared in the same vertical plane and collided with each other at an angle that was not 90 degrees. A digital SLR camera was maintained in direction perpendicular to the plane containing the jets to record the experiments, as shown in Figure 1.


Figure 1. When the velocity of two silicone oil jets increases from 0.53 meters per second (shown on the left) to 0.64 meters per second (shown on the right), the two jets will transition from bouncing to coalescence. (Courtesy of Virginia Tech)

The researchers adjusted the diameter and velocity of the jet flow and measured their contact length and the angle between the jet and a line perpendicular to the horizontal normal. Jung says, “We were surprised to find out that the contact time the jets are together in the collision area is independent of their velocity. The reason we are surprised is that if a liquid jet is aimed at a stationary wall, then it will stay longer on the wall as the velocity increases.”

Velocity is the key parameter that determines whether the jets bounce off each other or coalesce. When two silicone oil jets with diameters of 0.97 millimeter and a speed of 0.53 meters per second collide against each other, they bounce off. By increasing the speed to 0.64 meters per second while keeping all other parameters constant, the two fluids will now coalesce. Images of both are seen in Figure 1. The same phenomenon is also seen with the glycerol solutions. Fluid viscosity does not have an impact on whether the fluids bounce or coalesce.

The researchers determined that the transition from bouncing to coalescence can be expressed as a dimensionless parameter, K, which is a function of the Weber Number (the ratio of inertia to surface tension), the Reynolds Number (the ratio of inertia to viscosity) and the angle at which the jets collide with each other.

Jung speculates about what happens to cause the transition to coalescence. He says, “The air film between the jets becomes thinner as velocity increases, leading to instability at a critical thickness, which enables the force to be overcome and the two jets to combine.”

Jung believes that this research may prove to be useful in developing improved microfluidic channels. Additional information can be found in a recently published article (2) or by contacting Jung at sunnyjsh@vt.edu.

REFERENCES
1. Canter, N. (2012), “An Alternative Approach to Mixing Materials,” TLT, 68 (10), pp. 14-15.
2. Wadhwa, N., Vlachos, P. and Jung, S. (2013), “Noncoalescence in the Oblique Collision of Fluid Jets,” Physical Review Letters, 110 (12), p. 124502.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.