Drops and Emulsions moving on engineered surfaces

Seminario: Drops and Emulsions moving on engineered surfaces

Lunedì 20 Novembre
in Aula Malquori alle ore 15:00
il Prof. Matteo Pierno, Università di Padova, Dipartimento di Fisica e Astronomia “G. Galilei”,

Locandina

Abstract
The motion of drops on a solid surface attracts a lot of attention for its implications on
microfluidics and wetting [1]. On a tilted surface, this motion is the result of a balance between the
down-plane component of the drop weight and the viscous resistance, plus a capillary force related
to the nonuniformity of the contact angle along the drop perimeter [2]. On Slippery Lubricated
Surfaces (LIS) a suitable low surface tension lubricating liquid is trapped inside a surface texture [3].
This allows drops of an immiscible fluid to float on the lubricant layer with low friction, even for
highly viscous solutions which otherwise would hardly move on solid surfaces [4]. We report the
gravity-induced motion of small viscoelastic drops deposited on inclined lubricated surfaces (see
Figure, left panels). Viscoelastic fluids made of Polyacrylamide (PAA) and Xanthan gum of sufficiently
high N1 move down the tilted plane with an oscillating instantaneous speed whose frequency is
found to be directly proportional to the average speed and inversely to the drop volume [5].
When emulsion drops are confined within microfluidic channel, their flow occurs via
successive elastic deformations and plastic rearrangements which create fragile regions enhancing
the “fluidization” of the material. Although it is known that the fluidization is strongly affected by
the surface texturing, the role played by the density, the orientation and the periodicity of the rough
elements has not been fully addressed [6,7]. We report experimentally and numerically the flow of
concentrated emulsions in microfluidic channels, one wall patterned with grooves having different patterns on the length scale of the droplets
(see Figure, right panels) showing the evidence of different fluidization scenarios and directional effects [8-10].

References
[1] G. Mistura, and M. Pierno, Advanced Physics X 2, 591 (2017).
[2] P.G. De Gennes, F. Brochard-Wyart. and D. Quéré, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves. (Springer, 2004).
[3] T. S. Wong, et al., Nature 477, 443 (2011).
[4] S. Varagnolo, et al., Soft Matter 13, 3116 (2017)
[5] P. Sartori, et al., Communication Physics 5, 81 (2022)
[6] V. Mansard et al. Soft Matter 10, 6984-6989 (2014)
[7] J. Goyon et al., Nature 454, 84-87 (2008)
[8] L. Derzsi et al., Phys. Rev. E 95, 052602 (2017)
[9] L. Derzsi et al., Soft Matter 14, 1088-1093 (2018)
[10] D. Filippi et al., Adv. Mat. Tech., 8, 2201748 (2023)