“The one exclusive sign of thorough knowledge is the power of teaching”. A quote inspired from Aristotle holds true for today’s guest speaker who is a great inspiration for everyone across the globe.
On the 11th of August 2020 , we the students of Class 11 of Mushtifund Aryaans Higher Secondary School (MAHSS) were bestowed with the opportunity and privilege to interact with Professor Cameron Tropea. Also present for the session was Prof. Rajkumar Pant. A huge thanks to both for spending some of your valuable time with us. We are also very grateful to our Director Vyankatesh Prabhudesai for his relentless efforts to conduct this knowledge packed session.
Sir VPD commenced the session with a brief introduction of Prof. Cameron Tropea. Prof. Tropea did his Bachelors in engineering science, Masters degree in mechanical engineering from University of Toronto, Canada. He did his doctorate of engineering in civil engineering from University of Karlsruhe, Germany and his degree of habilitation in chemical engineering from University of Erlangen, South Germany. He started as a visiting assistant professor with the mechanical engineering department at the University of Waterloo, Canada. He then worked for a year as a general manager for a company named Entwicklung,Germany. From 1992 to 1997 he worked as a professor of fluid mechanics at the University of Erlangen, Germany. From 1997 till 2020, he worked as a professor of fluid mechanics and aerodynamics in the mechanical wing department of Technical University of Darmstadt , Germany. His research interest includes optical measurement techniques in fluid mechanics, interfacial transport phenomena, atomisation and spray processes and unsteady aerodynamics. He has taught various courses related to all these subjects at various universities. Prof. has won many awards and has guided around 100 PhDs. He is also a google scholar with 17900 editions with H- index 58. Sir VPD then requested the professor to enlighten the students with some of his brilliant research work.
Prof. then started his session and said that he felt it better to give a talk on all the subjects they do research in. He then shared his screen showing the topic “Physics of drops and sprays“. Mostly when we speak about fluid mechanics most people think of aerodynamics, civil engineering and hydraulics but this topic of drops and sprays is a wide field and has industrial relevance too. This is the main topic they work together with IISC Bangalore where they look at atomisation of metallic liquid metal that makes metallic powders which is used in 3D printing machines. He has also worked on the same topic at IIT Chennai. This is specially for injection in fuel nozzles in internal combustion engines.
He was very kind to appreciate and share the visuals of all the people who have helped him and provided financial aid for the research. He then started his presentation by showing examples where drop and spray effect is used:
1) Trains: droplets and rivulets going down the window
2) Inkjet printing
3) Fuel injection
4) Spray cooling and cleaning: has a wide application in spray effect.
He gave a few examples where spray nozzles are used:
- a) big tractors have these spray nozzles. These artificial spray nozzles are used
in agriculture fields and they are called flat fan nozzles. - b) On the windshields in a car, there is a nozzle that keeps oscillating back and
Prof. explained this with the help of a visual representation.
c)The car manufacturers also use the same phenomena to paint the car using
electrostatic rotary Bell atomizers. They use two types of atomizers:
- i) Rotary bell atomizers: have very highly charged cathode rays. Here the
efficiency is very poor. - ii) Electrostatic charged atomizers: your efficiency achieved is well over 96%.
Prof. then showed us the visual of a droplet impinging on a surface and interestingly the droplet moved upstream that is against the force of gravity. This is used in the exhaust system of trucks and cars. Here, the spray is used to put in urea and the exhaust systems clean-up the impurities.
Professor was very enthusiastic to explain the students regarding cameras and optics. He said that if one wants to capture the phenomena with a very high resolution then one should have a brief idea about focus, numerical aperture and circles of confusion. Most hobby photographers are very familiar with these concepts. Professor explained to us how the thin lens formula plays a very important role in the world of optics. It states that if F is the focal length of the lens, S is the distance of the lens from the object and S* is the distance of the lens from the film/ sensor then the formula is: 1/S* + 1/S = 1/F.
He also mentioned about the two magnification factors:
1) Transverse
2) Longitudinal
Professor focused on the point how the working distance of the lens changes with the change in other factors like the microscope and shape of the lens. Long distance microscope gives a high magnification but a large working distance. If you have a small lens you won’t get a lot of light intensity so one has to use a high sensitive sensor. But on the other hand if you use a big lens one can go far away and still get the same numerical aperture, collecting a solid angle and getting a lot of intensity.
In the last 15 years the development with cameras and also the mobile phone camera is just incredible. The mobile phones in Germany are called Handy. Making a comparison between the phones and the high speed cameras, he apprised us that the cameras have a higher resolution and can capture 20000 frames per sec whereas the mobile phone camera will capture around 150 frames per second that is 4 megapixels. Now one can buy a camera with very high spatial resolution and temporal resolution for a reasonable price.
Answering to one of the questions raised by professor R.K. Pant regarding the selfies clicked on mobile phones, Prof. Tropea replied that if the object is placed in some specific position the point would be out of the focus. Before the point reaches out of the focus, the image keeps on getting blurred as you move the object farther away from the lens. He said that to capture an image wherein one wants to focus on two objects, a Pinhole Aperture lens must be used so that the focal length is infinity. But if one uses a higher aperture lens the focal length gets smaller and smaller and one of the images appears to be blurred.
Typically in the research work a lens with very small depth of field and a large aperture is used.
He suggested the students read the book ‘Optics from Hecht’.
Prof. Tropea went on to discuss about Capillary forces, wetting and dewetting effects. Although this course is tremendously important and fascinating, it is not a part of the university lectures. He then discussed surface tension. There are several ways to describe it: in terms of energy or force.
Giving us a further explanation, Prof. said everytime we take water and want to increase its surface area; some energy is needed. Thus, this is the surface energy. A sphere has the lowest possible surface area and therefore requires least energy to maintain its shape. Water has the highest surface tension of 70.88N/m. Surface tension of other hydrocarbons is 20 to 30 N/m. By giving an illustration of a free body problem, he made sure we understood the force tension. Prof stated that a fog droplet of one micron has and atmospheric pressure of 1.457 bar. The pressure is higher than the outside since the curved surface leads to a jump in pressure this is called Laplace pressure. He also described the dynamics about the liquid sheet which does not have the 90 degree corners but instead gets a rim. If you have an infinitely small radius, the pressure will be very high which is nearly impossible. He also discussed conservation of momentum. The momentum equation balances the inertia of liquid entering the drop and the capillary forces. The thickness of the liquid sheet depends on the speed and the surface tension.
Prof. Tropea showed us a slide about a very high spatial and temporal resolution of atomisation. Here, with his experimental data he explained how an atomization nozzle would require 4.3 bar to atomise 1 litre of water into 1 micron drops. The large surface area improves heat and mass transfer. In vaporization and spray drying of milk products and insecticide powders we do atomization to increase the surface area. The efficiency is about 0.01%. That is only 0.01% energy is used to increase the surface area. He also mentioned that Chennai and Bangalore are the hotspots of atomization in India.
When one student asked the professor if it would be possible for humans means to float on water just like water striders, to which Prof. Replied that floating requires two forces:
1) Weight
2) Buoyancy
The buoyant force is directly proportional to the density of the fluid in which the object is immersed. A person will be able to float with much ease in oceans which have a higher salt content than fresh water. If a person is not totally submerged it means there is an equilibrium between the buoyant force and the surface tension. This theory is used for building canoes, sailboats and ships.
Answering one interesting question about why the surface tension of soap solution is less than that of water, Prof. explained that soap have molecules called surfactants that migrate to the surface of the liquid and reduce the surface tension of the water which is highly sensitive to contamination. In some of the washing machines where soap solution is not added, Adjuvants are added which affect the surface tension and break up the liquid sheet into filament.
He also mentioned to us why hot water is used to wash clothes. He said that when water is heated to 100 degree celsius it gets converted to vapours and the surface tension disappears. Hotter the water lower is the surface tension thereby lowering the pressure. The water seeps into the clothes much easily through the tiny pores.
He demonstrated how saliva has long chain polymers which can be stretched into long filaments. This is the basis of developing nanofibers. Like non- woven mats in cars are made of very thin filaments of polymers which entangle to form a mat.
He explained the process of erosion of turbine blades and limestone landscape by drop impact pressure by precisely explaining the terms like deformation time, impact time and critical velocity. Drop impact on different solid surfaces has been studied extensively by Prof.Tropea. Depending on several material properties and dynamic parameters the impact results in deposition rebound or splashes.wetting and non wetting systems is quantified by the contact angle.
He further went on to discuss wetting of different types of surfaces like hydrophilic, hydrophobic (water repellent) ,super hydrophobic surfaces and also water droplets on inclined surfaces with different contact angles. If the contact angle is 90 degrees or more it is a hydrophobic surface and if it is lower than 90 degrees it is a hydrophilic surface. With slide visuals he explained the dynamics of a droplet on an inclined surface.
Sir VPD was curious to know about the ice formation phenomena. Ice crystals are super cool large droplets. One can cool water to about -40 degrees but it won’t freeze until the nucleus starts to freeze. Through theoretical and pictorial descriptions and illustrations in 2 dimensions, the professor explained the process of ice formation. In Ice formation, layer nucleation plays an important role with nucleation rates determined primarily by energies on faceted ice- water or ice-vapour interfaces.Ice formation occurs due to:
- a) Nucleation and spreading of a thin ice layer.
- b) Dendritic freezing of a drop.
- c) Freezing of the remaining bulk drop.
He systematically explained through some videos in slow motion making the concept even more clear for the students. He also said that the freezing of ice fixes the momentary shape of the liquid drop. Freezing morphology and velocity also depend on the velocity.
Originally from Canada, Prof. Tropea has lived and worked in Germany for many years. He said that technical German is very close to technical English since most of the vocabulary comes from the Latin and Greek words and both German and English use the same sources. To learn German perfectly and extremely fast one has to find the right friend Circle.
Aspiring engineers do spend some time studying in Germany. There are 2 to 3 special exchange programs each year for engineering students especially aeronautical engineers from India and Germany. When asked whether the students can do the Masters in Universities of Germany in English the professor replied “Jein!!” ( A combination of Ja( yes) and Nein( no). In Darmstadt University, only the masters of Aeronautical engineering is taught in English.
Professor highlighted the coordination of research work conducted in various countries simultaneously.
For some medical aspirants, he was happy to talk about Magnetic Resonance Imaging (MRI) also referred as Magnetic Resonance Velocimetry(MRV) which is a tremendously dynamic field. Here, a non-metallic fluid system is injected in the whole body, so one can measure the flow of velocity inside the body in just 20 minutes. Mostly done for cardiovascular systems.
“There is nothing more practical than a good theory” ~ Albert Einstein.
Professor shared with us that he enjoyed learning engineering science the most which is more of Physics and maths. “Learning mathematics is like a toolbox and without the toolbox the instrumentation doesn’t help you at all, you need the toolbox of mathematics to interpret and gain insight into what you have observed.” he said.
He ended the session with a motivational quote:
“Do not be tempted into observational science”
In the end, professor R.K. Pant thanked and appreciated Prof. Tropea for enlightening the young minds with such immense knowledge in the field of Physics. Miss Eashita Prabhudesai proposed the vote of thanks on behalf of the students and expressed her gratitude to Prof. Tropea.
Within a short span of time he exposed the students to a lot of concepts of physics and fluid dynamics. Indeed Professor Cameron Tropea has a captivating and intriguing personality ! Having the chance to interact with one of the top professors in the world and yet a very humble person is one of the best experiences of our lifetime.
Written by,
Eeshani N. Pai Kane
Class 11.