Journal ArticleDOI
The effect of particle wettability on the stick-slip motion of the contact line.
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TLDR
In this article, a model was developed to predict the number of particles required to pin the contact line based on the force balance of the hydrodynamic drag, interparticle interactions, and surface tension acting on the particles near the surface line with varying particle wettability.Abstract:
Contact line dynamics is crucial in determining the deposition patterns of evaporating colloidal droplets. Using high-speed interferometry, we directly observe the stick-slip motion of the contact line in situ and are able to resolve the instantaneous shape of the inkjet-printed, evaporating pico-liter drops containing nanoparticles of varying wettability. Integrated with post-mortem optical profilometry of the deposition patterns, the instantaneous particle volume fraction and hence the particle deposition rate can be determined. The results show that the stick-slip motion of the contact line is a strong function of the particle wettability. While the stick-slip motion is observed for nanoparticles that are less hydrophilic (i.e., particle contact angle θ ≈ 74° at the water-air interface), which results in a multiring deposition, a continuous receding of the contact line is observed for more hydrophilic nanoparticles (i.e., θ ≈ 34°), which leaves a single-ring pattern. A model is developed to predict the number of particles required to pin the contact line based on the force balance of the hydrodynamic drag, interparticle interactions, and surface tension acting on the particles near the contact line with varying particle wettability. A three-fold increase in the number of particles required for pinning is predicted when the particle wettability increases from the wetting angle of θ ≈ 74° to θ ≈ 34°. This finding explains why particles with greater wettability form a single-ring pattern and those with lower wettability form a multi-ring pattern. In addition, the particle deposition rate is found to depend on the particle wettability and vary with time.read more
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References
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Journal ArticleDOI
Capillary flow as the cause of ring stains from dried liquid drops
TL;DR: In this article, the authors ascribe the characteristic pattern of the deposition to a form of capillary flow in which pinning of the contact line of the drying drop ensures that liquid evaporating from the edge is replenished by liquid from the interior.
Journal ArticleDOI
Long-scale evolution of thin liquid films
TL;DR: In this article, a unified mathematical theory is presented that takes advantage of the disparity of the length scales and is based on the asymptotic procedure of reduction of the full set of governing equations and boundary conditions to a simplified, highly nonlinear, evolution equation or to a set of equations.
Journal ArticleDOI
Contact line deposits in an evaporating drop
TL;DR: A theory is described that predicts the flow velocity, the rate of growth of the ring, and the distribution of solute within the drop that is driven by the loss of solvent by evaporation and the geometrical constraint that the drop maintain an equilibrium droplet shape with a fixed boundary.
Journal ArticleDOI
Evaporation of a Sessile Droplet on a Substrate
Hua Hu and,Ronald G. Larson +1 more
TL;DR: In this article, the evaporation of a sessile droplet with a pinned contact line was investigated experimentally, by analytic theory and by computation using the finite element method (FEM).
Journal ArticleDOI
Pattern formation in drying drops
TL;DR: The drop itself can generate one of the essential conditions for ring formation to occur: contact line pinning, and it is shown that when self-induced pinning is the only source of pinning an array of patterns-that include cellular and lamellar structures, sawtooth patterns, and Sierpinski gaskets-arises from the competition between dewetting and contact linePinning.
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