Analysis and optimisation of mixing in binary droplet collisions

Abstract

The collision of binary droplets plays a key role in several industrial, chemical and biological processes. In these processes, the quality of the desired outcome is strongly dependent on the mixing of the liquid droplets as they collide in mid-air. In this work, multiphase direct numerical simulations based on the volume-of-fluid method have been used to investigate the process of mixing and analyse the effects of parameters such as injection velocity, timing and collision angles. The evolution of mixing due to convection and irreversible diffusive processes has been quantified by means of the segregation parameter. To synthesise the outcome of a collision, the impact parameter has been redefined to account for the collision of non-spherical droplets. It has been found that the optimal mixing does not occur for symmetric head-on collisions, but rather at moderately asymmetrical configurations. This behaviour has been explained by analysing the velocity gradient tensor. It has been demonstrated that by breaking the symmetry, the local topology of the flow is altered and the resulting convective flows increase the contact area between the liquids, thereby augmenting the mixing process. However, it was also observed that lateral misalignment transforms the initial kinetic energy into the spinning of the merged droplets, thus preventing an enhanced mixing.

Binary droplet collisions in bioprinting: Influence of material properties on mixing and repeatability

“…As documented in a previous analysis [6], if the contact surface area lies on the reflection symmetry plane x = 0, the segregation parameter has a high value. Therefore, a large value of the segregation parameter is expected to occur at ν 1 /ν 2 = 1 as both liquids are identical and have been injected with identical conditions.…”

“…11 shows that the segregation parameter is the highest for M a = 0, which occurs for equal surface tension between the two liquids. This effect arises, as discussed in the previous subsection, due to the reflection symmetry in the system, forcing the contact surface to remain in the plane of symmetry and thus hampering the mixing of the liquids [6]. However, it can be observed that even small differences in surface tension produce large Marangoni numbers due to the small value of the diffusivity.…”

“…In this work, the methodology detailed in Ruiz-Gutiérrez, et al [6] is followed. The system of equations is solved using a second-order scheme for both temporal advancement and spatial discretisation in OpenFOAM by a finite-volume numerical method [17].…”

“…Other factors, such as the impact parameter, also play an important role. The effects of the impact parameter on the quality of mixing have been addressed by Ruiz-Gutiérrez, et al [6], and thus, will not be discussed here; the interested readers are encouraged to follow the reference for further discussion in this regard.…”

“…In the research area of binary droplet collision, much progress has been accomplished in predicting the coalescence and separation in the aftermath of the collision, however, the process of mixing as the two liquids combine has just begun to be explored [1,6]. Moreover, for technologies such as bioprinting, the quality of the mixing is key to ensuring the viability and consistency of the printed tissue.…”