An unexpected journey: From simple colloids to mutually coupled superstructures

This is the story about how we came to discover a complete new material system starting with only basic building blocks and light as our activation switch. The route taken has not been obvious to us from the beginning; we needed creativity and persistence to expand our previous knowledge.

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It all happened unexpectedly. Back in 2018 I had just finalized my experimental setup and started playing around with the tools that were available to me; using light and a binary mixture of colloidal particles suspended inside a critical environment. Surprisingly, I have found that under the right combination of light and the temperature-sensitive critical mixture, single colloids combined in pairs forming a new type of structure that resembled molecules. Through the combination of light-absorbing and non-absorbing colloids, previously diffusing only passively, were now activated under light illumination forming Janus dimers (for the greek god with two faces). Over time these structures grew and formed more complex and even chiral colloidal molecules that exhibited various interesting types of motion. Importantly, this system self-assembled from simple building blocks into active structures that help us better understand how naturally systems have evolved and enable us to build new artificial materials using a bottom-up approach.
But it didn't end there.

Occasionally, when I haven't been carefully controlling the system's parameters those colloidal molecules grew larger and larger over time. Eventually, a significant amount of absorbing colloids would create such an immense temperature increase that the surrounding critical solution would demix and form an all encapsulating droplet. I dismissed most of these cases as all molecular structures were diminished in the process and no activity could be observed any longer. I continued with my experiments, but in the back of my mind, curiosity about this process grew over time, as did my molecules, and I eventually revisited this fringe phenomena. After discussing this with my co-authors, we started to see the potential of this system. There was something intrinsically new about this droplet formation process that we couldn't really put our fingers on, yet.

Figure 1 Overview of the self-assembled structures in chronological order as they have been discovered. Left: Single colloidal molecules made of absorbing (red) and non-absorbing (blue) colloids show complex active motion in their trajectories. Middle: With time a large number of absorbing colloids accumulates that eventually leads to droplet formation (blue shadow in background) and a breaking of the molecular structures. Right: At ideal settings, active molecules remain which drives the droplet as well such that active droploids emerge.

The following months have been tough. I tried to systematically replicate these occasional phenomena without much success. When every reasonable approach seemed to fail, creativity (with a mix of frustration) kicked in and a seemingly wild guess to purposely prepare the solution away from its delicate criticality showed first signs into the right direction. In the meantime our co-authors build up a simulation model trying to replicate those experimental findings. After a couple of rounds iterating the model with experimental data we were able to simulate droplet formation and finally held the key to its working principle in our hands. As we have intuitively guessed before, our solution had to be prepared off-critical, which creates the necessary energy barrier to induce such local phase separation. A seemingly small offset of only 2% is enough, which was previously introduced only due to human errors in sample preparation. Now, we were finally in a position to gain a deeper understanding of our system.

We noticed in particular that not only had the absorbing colloids an effect on their environment, i.e. by local heating, but that the formation of the droplet around the colloids also influenced their structure. We called these new emerging structures "droploids", a portmanteau of the words droplet and colloids. In this way, a two way feedback loop was created in which colloids and droplet influence each other. This became especially prominent when enough heating through absorption was produced that droplets formed but not as much that molecules broke apart again (as in the original observation). In this case, colloidal molecules were still intact and active, moving the droplet as they were moving in space such that the droplet itself became active; thus called active droploids. This intrinsic feedback marks a new breakthrough in the creation of artificial materials as energy is not only flowing from environment to colloid (or vice versa) but both continuously influence each other, from which also parallels to natural systems such as in the compartmentalisation of the cytoplasm or the formation of membrane-free organelles can be drawn. Our insights could inspire new active-matter research using two-way feedback loops also in other systems and are the first steps towards a new generation of light-activated biomimetic materials.

uzF94V3XRyiQEJZr9gbY_gartner hype cycle.jpg
Figure 2 Adaptation of the Gartner Hype Cycle to our expectations vs. the progress we made along the way. From the start of the experiments till the observation of new phenomena expectations quickly peaked before plummeting to a minimum while unsuccessfully trying to reproduce them. After extensively exploring the system in simulations and experiments, breakthrough was made and lead to a plateau in which we had carefully studied and characterized our system.

It is now 2021, three years later since the start of the original project, and I have the chance to look back at our path taken (Fig. 2). Although in most scientific articles the hypothesis, the problem or the goal seemed clear from the beginning, this is hardly the reality of many researchers and it has certainly not been for us (in fact, it has been a rollercoaster of expectations vs. reality). However, without going through the process of excitement about observing a new phenomena, to the struggle of trying to reproduce such until the system is actually running again, one can hardly say that is has been clearly investigated or understood. Along the way we not only gained new insights on fundamental aspects of feedback systems but we could expand our analysis methods by using machine learning algorithms to be able to track and extract valuable information, for example, on droplet speed, size and shape.
All of this was only possible, by staying curious and be persistent in trying to push the boundaries of what is possible even further.

Until eventually strange new fringe phenomena appear again, and the whole cycle starts from the beginning...

Link to the precursor work: doi.org/10.1063/1.5079861

Link to the present article in Nature Communications: doi.org/10.1038/s41467-021-26319-3

I want to thank all my co-authors for their enthusiasm, hard work and dedication throughout this project. Special thanks go to Jens Grauer for creating the simulations without we wouldn't have been able to explore this system in such great depth, to Benjamin Midtvedt and Jesus Pineda for their innovative machine learning method for tracking ever evolving structures, and Hartmut Löwen, Giovanni Volpe and Benno Liebchen for their invaluable feedback and support along the way.

Dr. Falko Schmidt

PostDoc and Coordinator of Scientific Research, Friedrich Schiller University Jena