Instabilities, motion and deformation of active fluid droplets
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| License | CC Attribution 3.0 Unported: You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor. | |
| Identifiers | 10.5446/38813 (DOI) | |
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51
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Transcript: English(auto-generated)
00:04
This paper presents work done at the University of Sheffield by myself, Karl Whitfield, and my PhD supervisor, Dr Rhoda Hawkins. Spontaneous movement and deformation are key mechanisms underlying many aspects of the animal cell cycle. However, these same mechanisms are often enhanced in metastatic cancer cells, making them more agile and more invasive.
00:23
These mechanisms are physically driven by the cell cytoskeleton. The cytoskeleton consists of protein filaments and motors which constantly consume chemical energy, ATP, and convert it to work. This is what we mean by an active material. In particular, active filaments interact with myosin motors to generate contraction forces in the cell, which can drive cell motion and division.
00:45
Recently, experimental techniques have allowed researchers to study these active materials in isolation by trapping them inside vesicles or droplets. In this work, we investigate two minimal models of the dynamics of these active droplets. The first we consider is where the cytoskeletal material coats the interior surface of a fluid droplet, as is often observed in experiments.
01:07
The contraction of the active material parallel to the interface changes the surface tension locally, and this can give rise to Marangoni flows. These can then pull in more active material, creating a feedback loop. We use linear perturbation theory to investigate when activity is strong enough to give rise to these instabilities, and what form they take.
01:24
At low activity, a single peak in concentration is formed, and the droplet swims, as you can see here. At higher activity, two peaks quickly collapse into one, forming the same state. The simulations also show that two peaks can be sustained through diffusion
01:41
and advection of the material through the droplet interior, shown here in orange. Secondly, we consider a drop with a high concentration of material, such that it forms an ordered liquid crystal phase. We model this like a passive liquid crystal emulsion, where the molecules are often strongly aligned at the interface. In this case, the force is generated along the direction of the filaments, which is strongly coupled to the shape of the drop by elastic free energy.
02:08
Using the same techniques, we show that if the stresses generated are contractile relative to the filament direction, this gives rise to a shape instability. Whereas if they are extensor, then we find that they break the translational symmetry. The shape instability depends crucially on the droplet surface tension, whereas this transition to motion is only governed by the internal dynamics.
02:28
While these two models are somewhat simplified compared to reality, we see that they both predict regimes of spontaneous motion and deformation as a robust feature of confining these materials to droplets. Furthermore, our predictions give quantitative insights into these dynamics, which
02:40
will inform future experimental work on these active droplet systems. Thank you for watching.
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