Polymers Under Flow

Polymer solutions play not only an important role in biology but also in our daily life. Polymer solutions are needed for the fabrication of numerous products and can be found in many foodstuffs and cosmetics. Despite their importance, there are still many open questions which need to be adressed. 

Transient changes in the conformation of the individual molecules, can hardly be resolved with bulk experiments. On the other hand it is difficult to determine bulk rheological properties from single molecule studies. It is this gap we aim to adress by introducing complex microfluidics. 

Have you seen such liquids?

What are the special properties we are looking at? 

We concentrate on the so called non-Newtonian fluid properties of these solutions – the speed of the fluid is not linear dependent on the pressure as we experience in our daily life for e.g. water. in addition the speed is even strongly depenent on the geometry it flows in! This has important consequences - not only for getting ketchup out of the tube, but also for spider silk spinning, processing in industry or blood flow in your body. We aim to build experimental tools to watch and understand the underlying physical principals.

Fig. 1 shows a highly concentrated starch solution which is put on a speaker. The speaker is driven with an sinus with a frequency of O(100Hz); Left: Video at normal speed, Right: Patternformation, video recorded with highspeed cam.

Fig. 2 Left: Rod-climbing of the same polymer solution. The liquid climbs up a turning rod! Right: The open siphon effect (=flow UPwards!): Due to the non-linear behaviour of the solution (mixture of water, glycerin, polymers and ink), the liquid can be pulled into the syringe even when the syringe is lifted up (no fake!)!


Work in the labs

Besides the fun you can have with these polymer solution, there's a lot of research to do. Therefore you need the appropriate equipment.

Fig. 3 Photograph of a microfluidic device as used in our laboratory. They are made of PDMS (Poly­Di­Methyl­Siloxane) which is sealed on a microscope glass slide. We can build these devices in our labs. 

Fig. 4 is a photograph of a typical setup for microfluidics. The standard equiment is a microscope with flourescent lamp (green light), syringe pumps for pressure creation, specialized cameras (high speed, high sensitivity), pressure transducers and a pc for the data acquisition.

Fig. 5 shows the principles of different types of rheometers. Left: Shear rheometer, a cone is rotating against a plate. The force that is needed to achive a certain turning speed allows the calculation of the viscosity of a fluid. Right: elongational rheometer (=CaBER), the fluid is inserted between two plates which are then separated. The change of the fluid mid-point radius allows the calculation of relaxation times in polymer solution.