Dynamic Control of Particle Deposition in Evaporating Droplets by an External Point Source of Vapor

The deposition of particles on a surface by an evaporating sessile droplet is important for phenomena as diverse as printing, thin-film deposition, and self-assembly. The shape of the final deposit depends on the flows within the droplet during evaporation. These flows are typically determined at the onset of the process by the intrinsic physical, chemical, and geometrical properties of the droplet and its environment. Here, we demonstrate deterministic emergence and real-time control of Marangoni flows within the evaporating droplet by an external point source of vapor. By varying the source location, we can modulate these flows in space and time to pattern colloids on surfaces in a controllable manner.


Materials and Methods
Unless stated otherwise, each evaporation experiment was performed by depositing a 1-µL droplet of a 1-wt% water suspension of 2-µm monodisperse silica particles (Microparticles GmbH, SiO2-R-L1561) on a clean glass slide (VWR, 631-0113). A fresh particle suspension was prepared before each experiment in deionized (DI) water (resistivity > 18 MΩ.cm). To eliminate all source of contamination that could influence droplet spreading or pinning on the substrate, the slide was cleaned by sequentially sonicating it in acetone (Aldrich, > 99.8%), ethanol (Fisher, > 99.8%) and DI water for 5 minutes each. It was then immersed in a 1-M NaOH solution for 10 minutes, followed by a 5-minute sonication in DI water, and re-immersed in a 1-M HCl solution for 10 minutes, followed by sonication in DI water for 5 minutes for three consecutive times.
The slide was finally dried by withdrawing it from the water bath while exposing its surface to ethanol vapor (Marangoni drying). 1 After drying, the slide was placed on a homemade inverted microscope equipped with a CMOS camera for imaging (Thorlabs, DCC1545M). A custom-made environmental chamber (Okolab) enclosed the microscope to control temperature (T = 25 ± 0.2 °C) and relative humidity (RH = 45 ± 5%). The entire setup was mounted on a floated optical table in order to reduce vibrations. At the start of each experiment, the droplet was gently deposited on the slide with a graduated pipette using a disposable low-retention pipette tip (Brand,Z740080) and centered under the point source of vapor using the microscope stage. The point source consisted of a glass capillary (1.3 mm inner diameter) terminated with a blunt metal needle at one end. The needle was fixed to the capillary with a silicone-based sealant. The opposite end contained 10 µL of anhydrous ethanol (Aldrich, > 99.5%) held in place through capillary forces by sealing the open end with wax. Finally, the capillary was fixed to a three-axis motorized micrometric stage (Thorlabs, Z812B actuators with 0.2-µm minimum step) for fine positioning. To determine the distance between the needle and the substrate, a periscope together with a flip mirror allowed us to switch optical path so as to visualize the droplet's side view instead of its basal plane on the CMOS camera.

Calculation of the Sedimentation Velocity ‫ܛ‬
The sedimentation velocity of the 2-µm silica microparticles in water can be calculated using the following formula which takes into account the particle's gravitational force, drag force and buoyancy: where ܽ and ߩ ୗ୧ are the microparticles' radius and density, ߩ ୵ and ߟ are the density and viscosity of water, and ݃ is the gravitational acceleration. Figure

Supporting Movies
Supporting Movie 1: Movie of the evaporation process of the droplet corresponding to the final deposit in Fig. 1(b) showing the standard coffee ring effect. The video was recorded under a dark-field microscope. At the end a bright-field image of the final stain is shown.
the final deposit in Fig. 1(c) showing the formation of a deposit at the center due to the presence of an ethanol vapor point-source. The video was recorded under a dark-field microscope. At the end a bright-field image of the final stain is shown.