TY - JOUR
T1 - Evaporating droplets on oil-wetted surfaces
T2 - Suppression of the coffee-stain effect
AU - Li, Yaxing
AU - Diddens, Christian
AU - Segers, Tim
AU - Wijshoff, Herman
AU - Versluis, Michel
AU - Lohse, Detlef
PY - 2020/7/21
Y1 - 2020/7/21
N2 - The evaporation of suspension droplets is the underlying mechanism in many surface-coating and surface-patterning applications. However, the uniformity of the final deposit suffers from the coffee-stain effect caused by contact line pinning. Here, we show that control over particle deposition can be achieved through droplet evaporation on oil-wetted hydrophilic surfaces. We demonstrate by flow visualization, theory, and numerics that the final deposit of the particles is governed by the coupling of the flow field in the evaporating droplet, the movement of its contact line, and the wetting state of the thin film surrounding the droplet. We show that the dynamics of the contact line can be tuned through the addition of a surfactant, thereby controlling the surface energies, which then leads to control over the final particle deposit. We also obtain an analytical expression for the radial velocity profile which reflects the hindering of the evaporation at the rim of the droplet by the nonvolatile oil meniscus, preventing flow toward the contact line, thus suppressing the coffee-stain effect. Finally, we confirm our physical interpretation by numerical simulations that are in qualitative agreement with the experiment.
AB - The evaporation of suspension droplets is the underlying mechanism in many surface-coating and surface-patterning applications. However, the uniformity of the final deposit suffers from the coffee-stain effect caused by contact line pinning. Here, we show that control over particle deposition can be achieved through droplet evaporation on oil-wetted hydrophilic surfaces. We demonstrate by flow visualization, theory, and numerics that the final deposit of the particles is governed by the coupling of the flow field in the evaporating droplet, the movement of its contact line, and the wetting state of the thin film surrounding the droplet. We show that the dynamics of the contact line can be tuned through the addition of a surfactant, thereby controlling the surface energies, which then leads to control over the final particle deposit. We also obtain an analytical expression for the radial velocity profile which reflects the hindering of the evaporation at the rim of the droplet by the nonvolatile oil meniscus, preventing flow toward the contact line, thus suppressing the coffee-stain effect. Finally, we confirm our physical interpretation by numerical simulations that are in qualitative agreement with the experiment.
KW - coffee-stain effect
KW - contact line dynamics
KW - evaporation
KW - oil-wetted surface
UR - http://www.scopus.com/inward/record.url?scp=85088881288&partnerID=8YFLogxK
U2 - 10.1073/pnas.2006153117
DO - 10.1073/pnas.2006153117
M3 - Article
C2 - 32616571
AN - SCOPUS:85088881288
SN - 0027-8424
VL - 117
SP - 16756
EP - 16763
JO - Proceedings of the National Academy of Sciences of the United States of America (PNAS)
JF - Proceedings of the National Academy of Sciences of the United States of America (PNAS)
IS - 29
ER -