Binary Droplet Collisions in Bioprinting: Influence of Material Properties on Mixing and Repeatability

Abstract The study of suspended binary droplet collisions is an active research topic that has gathered interest due to its complexity and its industrial applications such as bioprinting. For many techniques that rely on the collision of two droplets, the quality of the outcome depends on the mixing process that begins when the droplets come in contact with each other. In this work, we study how the difference in material properties of colliding droplets, such as viscosity and surface tension, affect the mixing process. Employing multiphase direct numerical simulations, the distribution of the impinging liquids and the structure of the flow field have been analysed to find ways of mixing and process repeatability optimisation. To analyse the effects that differences in viscosity and surface tension have in the mixing process, the flows emerging from the collision are analysed. The differences in kinematic viscosity, in the range of $$1 \times 10^{-7}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>7</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> to $$1 \times 10^{-5}\,\text {m}^{2}\text {s}^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> <mml:mspace/> <mml:msup> <mml:mtext>m</mml:mtext> <mml:mn>2</mml:mn> </mml:msup> <mml:msup> <mml:mtext>s</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and surface tension, in the range of 36 to $${82.7\,\mathrm{\text {m}\text {N} \, \text {m}^{-1}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>82.7</mml:mn> <mml:mspace/> <mml:mrow> <mml:mtext>mN</mml:mtext> <mml:mspace/> <mml:msup> <mml:mtext>m</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> , have also been demonstrated to affect the trajectory of the centre of mass and morphology of the droplets upon collision and thus could have implications from the point of view of process repeatability. In the parameter space considered here, it was found that differences in viscosity and surface tension enhance the mixing due to the departure from the symmetry in the collision.