When Droplets and Particles Meet
TU Graz News: Can you describe your research for us?
Carole Planchette: My field of research is fluid mechanics, which is the physics of liquids and gases in motion. This field ranges from classic issues of aerodynamics in wind tunnels, including hydrodynamics, to complex technical flow systems.
In particular, I work on so-called multiphase flow. These are systems in which different phases – such as immiscible liquids, gases or solids – interact with each other. The behaviour of such systems is determined by the interactions between the individual phases. If these are finely dispersed – in the form of droplets, bubbles or particles – the interfaces become much more important, as the ratio of surface area to volume is high on these small scales. Specifically, this concerns droplets, bubbles, jets and small particles. Droplets or particle-laden interfaces serve as controllable model systems in which these mechanisms can be investigated with particular precision. At the same time, precisely these effects are relevant for many technical applications, for example in microfluidics, printing functional materials or coating processes.
I try to conduct research on the basics as well as keep an eye on the applied aspect. However, my focus is definitely on the basics. Because I am convinced that you have to fully understand a phenomenon before it can lead to a meaningful application.
Are you currently working on a project in this area?
Planchette: I am currently working with my group on several projects that investigate different aspects of multiphase flows.
One central project is “REMEDY”, an EIC Pathfinder project with international partners. We are trying to produce a so-called bio-ink from microorganisms. We want to print this ink precisely in small droplets on suitable panels. The printed elements will later be attached to façades, where they will form functional biofilms that bind CO2, for example. As these films are created from living microorganisms, they can close any minor damage through biological growth.
Together with Theresa Rienmüller from the Institute of Biomechanics and an industrial partner, I am also working on a lab-on-a-disk platform. A sample, blood for example, is applied to a rotating microfluidic chip in order to analyse a whole range of parameters from tiny amounts of liquid very quickly.
As part of the Collaborative Research Centre (CRC) on the optimisation of electric motors, coordinated at TU Graz by Annette Mütze and carried out jointly with TU Darmstadt, I am leading a sub-project in the field of cooling. Our contribution is currently starting and investigates a novel approach with so-called “compound” droplets, in other words, droplets made of two immiscible liquids. The aim is to combine the good wetting properties of the outer liquid with the efficient vaporisation of the inner liquid in order to dissipate heat in a targeted manner.
In another project backed by the Austrian Science Fund, I am investigating how droplets and particles collide in the air and what happens in the process. Does part of the droplet adhere to the particle? Is it enveloped or do they repel each other? Our work in this project is largely experimental.
What do you mean by that?
Planchette: It typically involves water or oil solutions. We want to better understand how critical such phenomena are in various industrial processes – for example in the production of milk or coffee powder by spray drying. At the same time, we want to find out what happens when the air is very dusty and then it rains. How much dust remains in the air and which particles are actually washed out?
How can you reproduce this in the laboratory? Do you make it rain there or do you stand in the courtyard in the rain?
Planchette: It’s not so easy. We have to ensure that droplets and particles collide in a controlled manner at a certain speed. That is why we produce both the droplets and the particles themselves. To do this, we use two droplet streams that meet in the air. One of these consists of a liquid that hardens under UV radiation – targeted irradiation produces solid particles that then collide with the liquid droplets. This gives us two precisely adjustable and reproducible currents so that we can systematically vary the collision conditions, from frontal to laterally offset collisions, with droplet diameters in the sub-millimetre range and speeds of up to ten metres per second.
How do you get your ideas for such experiments?
Planchette: The generation of uniform droplet flows is based on a classical physical instability, the so-called Rayleigh plateau instability. Such droplet generators are well established in fluid mechanics. However, the idea was to develop this principle further. We combine two such streams and use UV polymerisation to produce defined solid particles from one of them. In this way we can investigate droplet-particle collisions under precisely controlled conditions. This setup allows us to analyse fundamental interactions systematically.
Your research is mostly focused on basic research – why is that?
Planchette: My research is strongly focused on basic principles because I am convinced that technological developments can only be based on a sound understanding of physics. I’m not just interested in the fact that a phenomenon works, but why it works. Especially in fluid mechanics, many systems seem simple, but they are not. For me, the task is to develop suitable experiments in order to understand and model the underlying phenomena as well as possible.
How did you find your way into science?
Planchette: Even as a child I was interested in science and I was very curious. This curiosity has remained an inner driving force for me to this day. I could have opted for different scientific paths. During my studies in Paris at one of the “grandes écoles”, I came across inspiring people in fluid mechanics who really got me excited about this subject. It was then a very natural path for me to pursue this area scientifically. I completed my doctorate and then came to Austria for my postdoc.
What particularly fascinates me about fluid mechanics is the direct connection to everyday life. Many physical mechanisms manifest themselves in situations that we do not initially perceive as physical. For example, when coffee is mixed or when droplets stick to the shower wall and slowly slide down. Such seemingly simple observations in particular show how complex the underlying physical mechanisms are. It is precisely this connection from everyday life to physical theory and on to technology that I try to convey in teaching.
Is that how you see the world? So every time you stir your coffee or have a shower?
Planchette: Sometimes, yes. An example that I also like to use in teaching is the following: If you put a little water in a hot pan, the droplets start to roll over the surface. The water behaves completely differently than in a cold pan. This is precisely where the so-called Leidenfrost effect becomes very clear. Many people have already observed such phenomena – and it becomes exciting when you understand the physical principles behind them.
What plans do you have for your professorship?
Planchette: I have had a working group for some time, and we already had plans and visions before I accepted the professorship. Of course, I would like to continue working with this and expand the group in a targeted manner. I would like to integrate more people with different backgrounds to further strengthen the professional diversity in the team. I myself work mainly experimentally and develop my models analytically, but I see great potential in supplementing our work more with numerical simulation. The combination of experiment, modelling and simulation offers many new possibilities in our field.
Are there any particular research questions that you would love to answer?
Planchette: It is impossible for me to choose a specific question.
I find systems in which particles are involved at interfaces particularly exciting – whether they be droplets, bubbles or emulsions. Such systems can be found both in nature and in technical applications. The behaviour of the entire system often depends on what happens on a very small scale at the interface. I believe it is important to better understand these relationships – also in order to further develop existing processes. There are still many unanswered questions in this area.
