Session R19: Particle Laden

Effect of the colloid softness and concentration on droplet breakup and formation in microchannels

As a sustainable way to replace surfactants, colloids can be used to control formation of dispersed drops.

In this work, we investigate the formation of colloid-stabilised drops in a microchannel. Dodecane is used as outer phase, while water containing polystyrene particles (PS) or pNIPAM microgels (MG) is used as drop phase to study the effect of particle softness and concentration. Using high-speed imaging, the dynamics of the drop formation were capture for both particle types.

Following classic film breakup analysis, two trends were identified during the colloid-free drop formation: starting with an inertia-controlled neck thinning, it is the well-known competition between capillary and viscous forces that causes the neck breakup. The transition to the capillary- viscous regime happens when the neck reduces by approximately 30% of its maximum size. In the presence of MGs, the characteristic time to reach the capillary-viscous regime decreases independently of the concentration while PS particles shift gradually this transition with increasing concentration. However, in both cases the colloids affect significantly the initial inertia regime. Finally, just after breakup, higher deformations of colloid-laden drops were observed especially for high concentration of MGs which can imply a new elastic force component.   

Activity-Influenced Drop Evaporation

Active drops, by virtue of their capabilities to self-migrate, have found significant applications in disciplines ranging from designing novel emulsions to explaining bio-locomotion. This study discusses a theory of evaporation of active and nematic extensile drops. The interplay of activity and the mass loss due to evaporation enforces the air-liquid interface to descend and touch the solid substrate (where the drop rests). This results in the formation of punctured drops and the creation of an inner three-phase contact line and donut-shaped drop. A subsequent loss of liquid mass due to evaporation ensures that the contact angle at this inner contact line (ICL) becomes equal to the receding contact angle enforcing a runaway stage (drop dynamics with receding ICL, but pinned outer contact line or OCL) of the drops. The resulting particle deposition results in the formation of ring galaxy like pattern: the ring is at the location of the OCL, while there is a diffuse region of deposited particles following the trajectory of the receding ICL. We also discuss the cases where these phenomena are absent in active extensile drops, and the possible alterations in the drop evaporation lifetime between active extensile versus active contractile drops.     

Predicting Coalescence of Particle Encrusted Drops by Interfacial Shear Rheology

Food, pharmaceutical and cosmetic formulations are quite often Pickering type emulsions in which the interfaces of at least two immiscible liquids are stabilized by excipient particles which have crystallized out from one of the liquid phases. We demonstrate that the resistance provided by interfacial particle networks to coalescence can be predicted from coalescence depends the surface concentration of the particles laden dispersed droplets. The surface coverage concentration is uniquely determined by the emulsion making process. We treat this network as an elastic solid layer and interpret the onset of coalescence as to occur when the surface layer undergoes yielding under extension, we estimate the force on a droplet required to induce coalescence. Further, we extend this analysis to emulsions exposed to varying levels of centrifugal force and predict the amount of the dispersed phase recovered. We verify our model predictions by centrifuging the emulsions with different surface coverage and measuring the separated dispersed phases.

  

Diffusion across particle-laden interfaces in Pickering droplets

Emulsions stabilized by nanoparticles, known as Pickering emulsions, exhibit remarkable stability which enables many applications, from encapsulation, to advanced materials, to chemical conversion. The layer of nanoparticles at the interface of Pickering droplets forms a semi-permeable barrier between the two liquid phases, which can affect the rate of release of encapsulates, and the interfacial transfer of reactants and products in biphasic chemical conversion. The current lack of understanding of diffusion in multi-phase systems with particle-laden interfaces limits the optimal development of these applications. To address this gap, we developed an experimental approach for in-situ, real-time quantification of concentration fields in Pickering droplets in a Hele-Shaw geometry and investigated the effect of the layer of nanoparticles on diffusion of solute across a liquid-liquid interface. The experiments did not reveal a significant hindrance on the diffusion of solute across an interface densely covered by nanoparticles. We interpret this result using an unsteady diffusion model to predict the spatio-temporal effect of particles on diffusion across a particle- laden interface. We find that the concentration field of solute is only affected in the immediate vicinity of the layer of particles, where the area available for diffusion is affected by the particles. This defines a characteristic time scale for the problem, which is the time for diffusion across the layer of particles. The far-field concentration profile evolves towards that of a bare interface. This localized effect of the particle hindrance is not measurable in our experiments, which take place over a much longer time scale. Our model also predicts that the hindrance by particles can be more pronounced depending on the particle size and physicochemical properties of the liquids and can ultimately affect performance in applications.   

Unraveling Stress Relaxation of Cornstarch Droplet Impacting on Deep Pool

We recently studied the impact dynamics of droplets containing cornstarch particles on a deep water pool with varying Weber number and volume fractions of particles. We have found distinctive impact phenomena compared to Newtonian droplet impact, such as solid-like impact behavior. To better understand the origin of these phenomena, we performed rheology measurement to assess the stress relaxation of the cornstarch suspension. Specifically, different relaxation mechanisms have been observed with various volume fraction of particles and imposed shear rate, which could explain why liquid and solid phases of the droplet during impact process occurred. In addition, the relaxation timescale can be used to compare with the inertial time scale of the impact to further unravel the controlling physics of the distinctive impact phenomena observed in cornstarch droplet impact.

Brownian dynamics modeling of the assembly of electrosprayed particles on sessile droplet surfaces

Colloidal assembly at fluid interfaces is important in numerous chemical and biological processes. It also enables novel manufacturing of thin film materials. Electrospray is a nonintrusive method for delivering colloidal particles to a fluid interface such as the sessile droplet surface. A unique characteristic of electrospray targeting is imparting significant charge on both sprayed particles and receiving substrate. In this case, electrostatic interactions between particles and between particles and the substrate influence the dynamics and assembly structures at the interface. In this study, we investigated the effects of these electrostatic interactions on particle assembly on triangular and dumbbell-shaped sessile droplets using computational modeling. We combined ANSYS electrostatic simulation and a new Brownian dynamics algorithm to simulate charged particles moving on curved surfaces. Consistent with previous experimental observations, our simulations demonstrated the formation of a depletion region near the droplet contact line driven by the long-range electrostatic repulsion between particles and substrate. We quantitively characterized the effects of particle surface charge and particle concentration on the assembly structure and the depletion region development. The findings provide insights into the competition of various electrostatic interactions in the interfacial assembly of electrosprayed particles.   

Influence of wind on a viscous liquid film flowing down a thread

We investigate experimentally the effect of wind on the dynamics of a viscous liquid film flowing down a thread. The liquid film is well known to destabilize into an axisymmetric bead-like pattern in stagnant air. When the flowing thin film is exposed to side wind, the beads are pushed downstream of the wind, breaking the symmetry and leading to a modification of the film flow. The viscous dissipation in the liquid film decreases with increasing side wind, leading to a faster slide speed of the liquid beads. The inertia in the film is then increased, which can induce a regime shift, triggering an instability. When the film becomes unstable, it switches from a periodic pattern of equally sized liquid beads to an irregular film pattern where large beads collide with smaller ones.