Surface Morphology of Drying Latex Films: Multiple Ring Formation
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Surface morphology of drying latex films: multiple ring formation
Shmuylovich L, Shen AQ, Stone HA
Langmuir 18: 3441-3445 (2002)
The experiment was simple: a small drop of a monodisperse solution of particles (either 0.008% (w/w) solution of 0.88 um diameter particles, or 0.01% solution of 3.15 um particles) was placed on a glass microscope slide. The drying process was then captured on camera. As the solution evaporated it formed concentric rings of particle arrays.
It was observed that a larger drops size produced more rings that covered a wider area. Using solutions of larger particles decreased the number of rings, but did not have a significant effect on the diameter of the outer rings. Studying the dynamics of the contact line during liquid evaporation revealed that the motion is not continuous. Rather, the contact line became "pinned" and unable to move for periods of time, with different parts of the same ring being able to be pinned and unpinned at the same time.
Soft Matter Aspects
This paper presented data that explored the effects on contact line behavior of varying sizes of drops or particles. It also presents a qualitative, stochastic model to explain the observed contact line pinning. In brief, the contact line (the edge of the evaporating drop) moves until it runs into a particle (in fact, the higher rate of evaporation at the contact creates a convective flow that results in an outward flux of particles). A particle at the contact line "pins" the contact line, leading to an even greater evaporation rate and an even greater flux of particles towards the edge. The result of this is a concentrated array of particles that is left behind after the contact line continues to recede. These arrays take on the form of a 2D hexagonal crystaline structure.
The formation of ordered arrays by evaporating solutions of particles has many applications in material science. Of particular interest is the ability of dying particle suspension to form controlled 2D structures. This capability is of great interest for arraying biopolymers such as proteins. Though most biotechnological applications do not currently require precise spacial control during deposition, accurate patterning will gain importance as diagnostic and analytical assays continue to drive up throughput by increasing the density of deposited molecules.