Evolution of internal flows in mechanically oscillating sessile droplets undergoing evaporation

Multiscale vapor-mediateddendritic pattern formation and bacterial aggregation in complex respiratory biofluid droplets

HYPOTHESIS

Deposits of biofluid droplets on surfaces (such as respiratory droplets formed during an expiratory) are composed of water-based salt-protein solution that may also contain an infection (bacterial/viral). The final patterns of the deposit formed and bacterial aggregation on the deposits are dictated by the fluid composition and flow dynamics within the droplet.

EXPERIMENTS

This work reports the spatio-temporal, topological regulation of deposits of respiratory fluid droplets and control of bacterial aggregation by tweaking flow inside droplets using non-contact vapor-mediated interactions. Desiccated respiratory droplets form deposits with haphazard multiscale dendritic, cruciform-shaped precipitates when evaporated on a glass substrate. However, we showcase that short and long-range vapor-mediated interaction between the droplets can be used as a tool to control these deposits at nano-micro-millimeter scales. We morphologically control hierarchial dendrite size, orientation and subsequently suppress cruciform-shaped crystals by placing a droplet of ethanol in the vicinity of the biofluid droplet. Active living matter in respiratory fluids like bacteria is preferentially segregated and agglomerated without its viability and pathogenesis attenuation.

FINDINGS

The nucleation sites can be controlled via preferential transfer of solutes in the droplets; thus, achieving control over crystal occurrence, growth dynamics, and the final topology of the deposit. For the first time, we have experimentally presented a proof-of-concept to control the aggregation of live active matter like bacteria without any direct contact. The methodology can have ramifications in biomedical applications like disease detection and bacterial segregation.

Evaporation-Oscillation Driven Assembly: Microtailoring the Spatial Ordering of Particles in Sessile Droplets

This work explores the physical mechanism that can be used to control the final residual pattern of nanoparticles obtained from an evaporating-oscillating sessile droplet. To that end, the substrate is vibrated in the vertical direction with a constant amplitude, while the frequency of excitation is varied. It is found that evaporation progressively shifts the mode number of the oscillating droplet to lower values, while the oscillations enhance the rate of solvent loss, causing a reduction in the droplet lifetime. The coupling between evaporation and oscillation drives the internal flow through two distinct regimes. Initially, oscillation leads to inner flow recirculation, which delays the evaporation driven edge deposition of particles. Subsequently at lower modes, caused by solvent depletion, the effect of oscillation is weakened, which allows evaporation-driven flow to gain prominence and thus transport the dispersed particles to the contact line. We demonstrate here how this delay in particle migration can be controlled to engineer morphological changes in not just the resulting macroscopic aspect of the deposit but also its microstructure. We especially focus on the relatively unexplored microstructural pattern of deposits from evaporating-oscillating droplets. Using scanning electron micrograph and Voronoi tessellation of the final deposit, we show unique spatial variation in particle ordering at macro-micro length scales. Thus, droplet oscillation tunes the spatial extent of the particle ordering crucial in applications like photonic crystals and photonic glass.

Engineering Interfacial Processes at Mini-Micro-Nano Scales Using Sessile Droplet Architecture

Evaporating sessile functional droplets act as the fundamental building block that controls the cumulative outcome of many industrial and biological applications such as surface patterning, 3D printing, photonic crystals, and DNA sequencing, to name a few. Additionally, a drying single sessile droplet forms a high-throughput processing technique using low material volume which is especially suitable for medical diagnosis. A sessile droplet also provides an elementary platform to study and analyze fundamental interfacial processes at various length scales ranging from macroscopically observable wetting and evaporation to microfluidic transport to interparticle forces operating at a nanometric length scale. As an example, to ascertain the quality of 3D printing we must understand the fundamental interfacial processes at the droplet scale. In this article, we review the coupled physics of evaporation flow-contact-line-driven particle transport in sessile colloidal droplets and provide methodologies to control the same. Through natural alterations in droplet vaporization, one can change the evaporative pattern and contact line dynamics leading to internal flow which will modulate the final particle assembly in a nontrivial fashion. We further show that control over particle transport can also be exerted by external stimuli which can be thermal, mechanical oscillations, vapor confinement (walled or a fellow droplet), or chemical (surfactant-induced) in nature. For example, significant augmentation of an otherwise evaporation-driven particle transport in sessile droplets can be brought about simply through controlled interfacial oscillations. The ability to control the final morphologies by manipulating the governing interfacial mechanisms in the precursor stages of droplet drying makes it perfectly suitable for fabrication-, mixing-, and diagnostic-based applications.