In this work, we have demonstrated three unique regimes in the evaporation lifecycle of a pair of sessile droplets placed in variable proximity on a hydrophobic substrate. For small separation distance, the droplets undergo asymmetric spatiotemporal evaporation leading to contact angle hysteresis and suppressed vaporization. The reduced evaporation has been attributed quantitatively to the existence of a constrained vapor-rich dome between the two droplets. However, a dynamic decrease in the droplet radius due to solvent removal marks a return to symmetry in terms of evaporation and contact angle. We have described the variation in evaporation flux using a universal correction factor. We have also demonstrated the existence of a critical separation distance beyond which the droplets in the droplet pair do not affect each other. The results are crucial to a plethora of applications ranging from surface patterning to lab-on-a-chip devices.
Particle-laden droplet-based systems ranging from micro- to nanoscale have become increasingly popular in applications such as inkjet printing, pharmaceutics, nanoelectronics, and surface patterning. All such applications involve multidroplet arrays in which vapor-mediated interactions can significantly affect the evaporation dynamics and morphological topology of precipitates. A fundamental study was conducted on nanocolloidal paired droplets (droplets kept adjacent to each other as in an array) to understand the physics related to the evaporation dynamics, internal flow pattern, particle transport, and nanoparticle self-assembly, primarily using optical diagnostic techniques [such as micro-particle image velocimetry (μPIV)]. Paired droplets exhibit contact angle asymmetry, inhomogeneous contact line slip, and unique single-toroid microscale flow, which are unobserved in single droplets. Furthermore, nanoparticles self-assemble (at the nanoscale) to form a unique variable-thickness (microscale) tilted dome-shaped structure that eventually ruptures at an angle because of evaporation at a nanopore scale to form cavities (miniscale). The geometry and morphology of the dome can be further fine-tuned at a macro- to microscale by varying the initial particle concentration and substrate properties. This concept has been extended to a linear array of droplets to showcase how to custom design two-dimensional drop arrangements to create controlled surface patterns at multiple length scales.
We have deciphered that the vaporization rate of a pair of sessile droplets placed in a close vicinity of each other not only gets suppressed but also approached a universal pattern in the long time asymptotic limit, irrespective of substrate hydrophobicity. In a short time, these droplets exhibit a series of naturally evolving characteristics such as alteration of evaporation modes, flow transitions, asymmetric deformation, and motion of the contact line. Such dynamics are uniquely determined by the degree of pinning. In addition, we show that the enhanced hydrophobicity does not always lead to lower evaporation rate in droplets.
In this work, we provide a simple method to represent the contact line dynamics of an evaporating sessile droplet. As a droplet evaporates, two distinct contact line dynamics are observed. They are collectively known as modes of evaporation, namely Constant Contact Radius (CCR) and Constant Contact Angle (CCA). Another intermediate mode—Stick-Slide (SS) or mixed mode is also commonly observed. In this article, we are able to provide a graphical representation to these modes (named as MOE plot), which is visually more comprehensive especially for comparative studies. In addition, the method facilitates quantitative estimation for mode of evaporation (named as MOE fraction or MOEf), which doesn’t exist in literature. Thus, various substrates can now be compared based on mode of evaporation (or contact line dynamics), which are governed by fluid property and surface characteristics.
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