Vapor Mediated Interaction in Droplets

Abstract

Controlling internal flow in evaporating sessile droplets is desirable across applications ranging from lab-on-chip medical diagnostics, DNA profiling to surface patterning. Diffusion-limited evaporation in droplets exhibits very low internal flow velocities [∼O(10^−6) m/s]. Enhancement of internal flow is helpful for applications that demand in situ mixing at small-scale fluidic systems but are limited by the low Reynolds number. To overcome this limitation, we present a non-intrusive methodology to enhance flow inside the droplets without affecting their global evaporation pattern. A highly volatile ethanol droplet is positioned asymmetrically in the vicinity of a water droplet. The ethanol molecules are consequently adsorbed asymmetrically on the air-water interface creating a gradient in surface tension. This causes an internal Marangoni convection with flow rates ∼O(10^3) times higher than a naturally evaporating water droplet. The inter-droplet distance between ethanol-water is used as a control parameter to vary the strength of Marangoni convection. The flow pattern transitions through several regimes from asymmetric to symmetric double toroid once the ethanol droplet completely evaporates. Experimental flow visualization and quantification by micro-particle image velocimetry have been used alongside simple scaling arguments to quantify the physical mechanism at play. We can also switch between different flow patterns by strategic dispensing of ethanol droplets. Mixing at small fluidic length scales is especially challenging in viscous and non-volatile droplets frequently encountered in biochemical assays. In situ mixed methods, which depend on diffusion or evaporation-driven capillary flow, are typically slow and inefficient, while thermal or electro-capillary methods are either complicated to implement or may cause sample denaturing. As a consequence of increased velocity by vapor-mediated interactions, we can use it to enhance mixing in droplets. We demonstrate a decrease in mixing timescale in a sessile droplet of glycerol by simply introducing a droplet of ethanol in its near vicinity. The fast evaporation of ethanol introduces molecules in the proximity of the glycerol droplet, which is preferentially adsorbed (more on the side closer to ethanol), creating a gradient of surface tension driving the Marangoni convection in the droplet. We conclusively show that the mixing time reduces by ∼10 hours due to the vapor-mediated Marangoni convection for the given volume of the droplet. Simple scaling arguments are used to predict the enhancement of the mixing timescale. Experimental evidence obtained from fluorescence imaging is used to quantify mixing and validate the analytical results. This is the first proof of enhanced mixing in a viscous, sessile droplet using the vapor mediation technique. Further, sessile droplets of contrasting volatilities that can communicate via long-range (∼O(1) mm) vapor-mediated interactions are used to remote control the flow-driven self-assembly of nanoparticles in the drop of lower volatility. This allows morphological control of the buckling instability observed in evaporating nanofluid droplets. A nanofluid droplet is dispensed adjacent to an ethanol droplet. Asymmetrical adsorption-induced Marangoni flow (∼O(1) mm/s) internally segregates the particle population. Particle aggregation occurs preferentially on one side of the droplet, leaving the other side to develop a relatively weaker shell that buckles under the effect of evaporation-driven capillary pressure. The inter-droplet distance is varied to demonstrate the effect on the precipitate shape (flatter to dome-shaped) and the location of the buckling (top to side). In addition to being a simple template for hierarchical self-assembly, the presented exposition also promises to enhance mixing rates in droplet-based bioassays with minimal contamination. Vapor-mediated interaction in droplets can have implications in controlling agglomeration in functional droplets. A functional sessile droplet containing buoyant colloids (ubiquitous in applications like chemical sensors, drug delivery systems, and nanoreactors) forms self-assembled aggregates. The particles initially dispersed over the entire drop-flocculates at the center. We attribute the formation of such aggregates to the finite radius of curvature of the drop and the buoyant nature of particles. Initially, larger particles rise to the top of the droplet (due to higher buoyancy force), and later the smaller particles join the league, leading to the graded size distribution of the central aggregate. This can be used to segregate polydisperse hollow spheres based on size. The proposed scaling analysis unveils insights into the distinctive particle transport during evaporation. However, the formation of prominent aggregates can be detrimental in applications like spray painting, pesticide industries, washing, coating, lubrication, etc. One way to avoid the central aggregate is to spread the droplets completely (contact angle ~ 00), thus theoretically creating an infinite radius of curvature leading to uniform deposition of buoyant particles. Practically, this requires a highly hydrophilic surface, and even a tiny inhomogeneity on the surface would pin the droplet giving it a finite radius of curvature. We demonstrate that using non-intrusive vapor mediated Marangoni convection higher than the evaporation-driven convection) can be vital to an efficient and on-demand manipulation of the suspended micro-objects. The interplay of surface tension and buoyancy force results in the transformation of flow inside the droplet leads to spatiotemporal disbanding of agglomeration at the center of the droplet. We also showcase a mechanism of asymmetric solvent depletion using vapor-mediated interaction that can non-intrusively regulate the site of crystal precipitation. In general, the flow pattern inside a drying sessile saline droplet leads to the circumferential deposition of salt crystals at the end of evaporation. Instead, we show that our proposed approach can manipulate the spatial location of crystal precipitation. The introduction of a pendant ethanol droplet near the sessile saline droplet’s vicinity creates an asymmetric ethanol vapor gradient around the sessile drop. The vigorous and non-uniform Marangoni flow promotes targeted contact line depinning, ensuring preferential segregation of the salt crystals. Using this methodology, we can inhibit crystal formation at selected locations and favorably control its deposition in definite regions. The interplay of flow hydrodynamics and the associated contact line motion governs this phenomenon marked by the inception and growth of crystals at a preferential site. The universal character of such a phenomenon is verified for a variety of salt solutions on the glass substrate. 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 are dictated by the composition of the fluid and flow dynamics within the droplet. This work reports the spatio-temporal, topological regulation of deposits of respiratory fluid droplets and control of motility of bacteria by tweaking flow inside droplets using non-contact vapor-mediated interactions. Respiratory droplets form multiscale dendritic, cruciform-shaped precipitates when evaporated on a glass substrate. However, we showcase that using non-intrusive vapor mediation as a tool can control these deposits at nano-micro-millimeter scales. We morphologically control dendrite orientation, size and subsequently suppress cruciform-shaped crystals. The nucleation sites are controlled via preferential transfer of solutes in the droplets; thus, achieving control over crystal occurrence and growth dynamics. As a result, active living matter in respiratory fluids like bacteria is preferentially segregated and agglomerated with controlled motility without attenuation of its viability and pathogenesis. For the first time, we have experimentally presented a proof-of-concept to control the motion of live active matter like bacteria in a near non-intrusive manner. The methodology can have ramifications in biomedical applications like disease detection and bacterial segregation.