We are developing a new method to to entrap water containing analytes in silicone based gels in a controlled manner: when water droplets are embedded in an elastic matrix, they shrink due to vapor diffusion through the (dry) polymeric matrix, which follows a slow diffusive-limited process which can be modeled and predicted accordingly. During the droplet shrinkage, the pressure in the liquid decreases as a consequence of the build up of elastic stress at its interface. This underpressure eventually leads to the birth of a cavitation bubble. Using high speed imaging, we access the dynamics of the bubble growth and subsequent oscillations. We show that these observations can be understood through a modified Rayleigh-Plesset equation to account for elasticity. The experiments reveal the cavity dynamics over the extremely disparate timescales of the process, spanning 9 orders of magnitude. Such a model system could serve as a new paradigm for motile synthetic materials.
A more sophisticated line of research involves the use of microfluidic devices, not only to generate the particle assembly, but also to monitor it under the microscopy at high-resolution. We have now setup a system in which we can evaporate droplets at a very controlled rate, and most importantly, to visualize their interior to see how the assembly of particles take place. We have studied how the evaporation/dissolution of droplets in such systems occur. Due to the presence of more complex boundary conditions, the process needed to be modelled numerically, which gives very good comparison with the theory. Small clusters of particles have been already produced, and we are currently working on studying how the assembly occurs with Brownian particles confined in the picoliter droplet.
Departing from spherically-shaped droplets, by a collaboration with micro/nano-fabrication groups at the University of Twente in NanoLab, we are currently exploring the possibility of depositing clusters of nanoparticles on micro-structured substrates. A much more practically simpler, but fundamentally more challenging is the particle assembly at droplet/air interfaces: this line of work relies on the adsorption of particles at liquid/air interfaces in order to create a monolayer of nanoparticles which can then be employed for detection techniques. The approach is completely different than the other lines. We have been recently studying how particles agglomerate at the interface during the evaporation of sessile droplets, and on how the presence of salts enhances such a process.
