Dynamics of active droplets and freely-jointed colloidal trimers

Abstract

In this thesis we have investigated how the dynamics of particle are affected by surface activity, which is the property of particles to locally alter the solute concentration through for example a surface reactions or dissolution. We found that surface activity can have three effects on the particle dynamics. First, it can cause the particles to self-propel. Surprisingly a heterogeneous surface activity is no prerequisite for this and also particles with isotropic surface activity can swim due to a hydrodynamic instability, provided the activity is larger than a threshold value. Particles that move due to this instability are called isotropic swimmers. In Chapter 2 we studied the swimming dynamics of droplet that slowly dissolve in surfactant solution as a model for such isotropic swimmers. We found that their persistence time can be tuned through droplet size and the surfactant concentration. This finding suggests that stochastic character in the motion of active materials on granular length scales is not only caused by Brownian rotation of these active particles. Rather we think that fluctuations in the fluid flow or spatial inhomogeneities in the dissolution rate cause stochastic turning. Second, we found that even below the onset of swimming, the dynamics of particles with homogeneous surface activity are enhanced or attenuated by the activity, depending on whether solute is consumed or produced. In Chapter 3 we investigated theoretically the instability that gives rise to self-propulsion for isotropic particles. We found that particles with a surface activity just below the swimming threshold can coast as if they were inertial, even though they are in the low Reynolds number regime. We made an attempt to test this finding experimentally, but the results remain inconclusive. Third, surface activity induces effective interactions between particles. We measured such solute-mediated interactions between two dissolving oil droplets in Chapter 4 and found that the interaction scales with inter-particle distance as $1/r^2$. Moreover the interaction strength increases with droplet size and surfactant concentration. Because solute-mediated interactions are dissipative and involve the solvent, they can have the unique property that particle 1 is attracted by particle 2, while particle 2 is repelled by particle 1. This asymmetry in solute-mediated interactions can lead to chemotactic chasing, when the interaction strengths are properly tuned, as we show in Chapter 5. We also show that clusters of chasing droplets can move translationally, rotationally or reorganize depending on their geometry. We made a step in the direction of applying the knowledge of the phenomena that we learned for dissolving droplets to solid colloids. The biggest hurdle standing in the way of that comparison is that the phoretic mobility for solid particles is unknown for many solute gradients and not easy to measure experimentally. In Chapter 6 we first reproduced earlier measurements of diffusiophoretic mobilities of solid particles using microfluidic devices. Then we set out to improve this technique so that it requires fewer particles and no longer relies on the particles being fluorescent.