Seeing the unseen

Evan Zabawski | TLT From the Editor March 2013

Detecting nearly invisible contamination.

 

“YOUR STAFF IS MIXING LUBRICANTS BEFORE THEY EVEN USE THEM,” I told a customer one day. He asked how I knew, mainly to provide evidence so that he could turn the problem into a learning experience. I told him that it was the Schlieren effect that tipped me off.

For that answer, I was remembering terminology I had learned 25 years earlier in a junior-high science class. The students were experimenting by dropping food coloring into vials of water at various heights. When the dye entered the water, it created the most amazing colored ribbons and swirls before the color became uniformly dispersed. Greater heights yielded greater depths of color penetration and a reduced time for homogeneity.

Schlieren (the German word for streaks) is defined as optical inhomogeneities in transparent material not visible to the human eye and is used to describe an observed phenomena—largely attributed to August Toepler, who was studying fluids flow in 1864. Schlieren can be observed by the naked eye as shadowgraphs (single, slow-speed imagery) and can be static or dynamic. A summer barbeque offers an opportunity to witness both variants: the shadow of an empty drinking glass upon a table (with all the variations of thickness appearing as shadows) or the shadow of a lit BBQ (shadows of turbulent plumes rise as if the BBQ was smoking profusely).

Schlieren photography (multiple frames of high-speed imagery) played an important role studying schlieren in supersonic and subsonic wind tunnels to measure aerodynamics. This imaging process reveals the interactions of the aircraft with its airspace, showing the turbulence and shockwaves created by various designs. They helped aero-dynamicists develop the swept back wing design seen in today’s fighter jets and the recently retired space shuttles, as well as the blunt body design used by the Mercury space program for its reentry vehicles.

The schlieren I witnessed in my science class was made possible because the two mediums (water and dye) were different colors and densities, whereas flaws in drinking glasses or thermal heat are invisible to the naked eye. Also, the interaction was very slow, unlike supersonic aerodynamics. For these reasons, I was also able to observe schlieren in the customer’s lube-oil transfer containers when I shone my flashlight into them.

In this case, the two mixed fluids were relatively comparable in color (the ubiquitous golden hue of so many hydraulic, pump and turbine oils at this plant). But as I tipped the container to shine my flashlight inside, the fluids began to shift and mix, and the bottom of the container suddenly had swirling shadows dancing on it. When I repeated this with another container to show the customer, he was not so easily convinced.

I asked him for a clear container and when we poured the suspected mixture into it, we no longer needed a flashlight to see the effect. Much like the dye-in-water experiment, these two fluids were swirling like intertwined ribbons before settling out into a reasonably homogenous mixture. That’s the problem—after the two similarly colored fluids are mixed together well, it is very difficult to see any separation. Once they are left undisturbed for a while, the varying densities of the two fluids cause stratification and give us another opportunity to observe the schlieren when remixed.

Dismayingly, many lubricants are of relatively equal density and even with some color disparity the opportunities to witness schlieren are fleeting, as they will permanently disappear since the two fluids will never stratify. This scenario can also be witnessed by those who add milk or creamer to their coffee. Schlieren are visible at first, but disappear quickly and forever.


Evan Zabawski, CLS, is a reliability specialist in Calgary, Alberta, Canada. You can reach him at evan.zabawski@gmail.com.