Taming the Coffee Ring Effect: Enhanced Thermal Control as a Method for Thin-Film Nanopatterning

Abstract

Predicting and controlling a droplet's behavior on surfaces is very complex due to several factors affecting its nature. These factors play a crucial role in colloidal material deposition and related solution-based manufacturing methods such as printing. A better understanding of the processes governing the droplet in the picoliter regime is needed to help develop novel thin-film manufacturing methods and improve the current ones. This study introduces the substrate temperature as a method to control the droplet's behavior during inkjet printing, especially the coffee-ring phenomena, at an unprecedented temperature range (25-250 °C). To explain the particular behavior of the droplet, this research associates the creation of specific coffee-ring micro/nanostructures at elevated temperatures with the Leidenfrost effect that is responsible for creating a vapor pocket under the drying drop. Herein, we combine experimental data and numerical methods to explain the drying dynamic of the picoliter-size droplet on the substrate at elevated temperatures. The achieved results indicate that the coffee-ring effect is correlated with the heat-transfer changes caused by the Leidenfrost effect and can be controlled and used to produce micro/nanostructured thin films without additional processing steps.

Figure 1

Graphical representation of the coffee-ring and Leidenfrost effects. (a) Coffee-ring pattern made by coffee and its profile after drying. The image and the profilometry results confirm that most of the material remains on the edge of the stain. (b) Three-dimensional fitting of the spherical surface for the concave-plano lens into an inkjet-printed micro/nanostructure that can be used in disposable microscopy diagnostics. (c) Demonstration of the Leidenfrost effect: the drop of water is hovering over the surface on the vapor cushion. (d) Leidenfrost heat-transfer curve: the colored areas indicate different regions of drop behavior: natural convection where the drop dries slowly, nucleate boiling area where the heat transfer increases significantly, transition boiling where the increasing temperature reduces the drop drying until reaching the Leidenfrost point, and film boiling where the sustained increase of the substrate temperature results in increased heat transfer and rapid drop evaporation.

Figure 2

PEDOT:PSS droplet’s coffee-ring behavior at various temperatures. The images represent the critical points of the substrate temperature from the perspective of the Leidenfrost effect: (a) Natural convection. (b) Nucleate boiling area where the critical heat flux occurs at 130 °C. (c) Transition boiling where the Leidenfrost effect arises with the Leidenfrost point at 220 °C. (d) Film boiling temperature range where heat transfer increases significantly above 240 °C. (e) Cross-sectional profiles of the droplets at the aforementioned points. The full spectrum of the cross-sectional profiles is demonstrated in Figure S3 in the Supporting Information. (f) Controlled behavior of the droplets of PEDOT:PSS produces a highly ordered nanopattern. The substrate temperature of 120 °C results in micro/nanostructures of approximately 50 μm in diameter and 45 nm in height. The continuity of the pattern proves that the droplets dry in their impact location. A single nozzle of the Dimatix printer is capable of printing at a speed of 5000 drops/s. Hence, 16 nozzles allow patterning at a speed of approximately 2 cm²/s.

Figure 3

Correlation between the Leidenfrost and coffee-ring effects for various solvents and materials. The standard deviation for each curve is presented as the shadow line. (a) Leidenfrost curve representing the heat transfer between the substrate at an elevated temperature and a droplet of water at the boiling point (BP) (100 °C). The dashed and dotted lines highlight two critical temperature points for water-based inks: the critical heat flux (130 °C) and the Leidenfrost point (220 °C). (b) Change in the depth of the coffee-ring structure for water-based PEDOT:PSS ink in the temperature range of 25–250 °C. (c) Coffee-ring behavior of the droplets at various temperatures for ZnO IPA ink. The horizontal arrow-lines indicate the shift of the curve (by 20 °C) toward lower temperatures caused by the lower boiling point of IPA (83 °C). (d) Coffee-ring behavior of the droplets at various temperatures for Cu TGME ink. The horizontal arrow indicates the shift of the curve (by approximately 140 °C) toward higher temperatures caused by the higher boiling point of TGME (249 °C). The green line shift is not depicted because its location is out of the range of our measurement setup (∼379 °C).

Figure 4

Numerical analysis of the droplet behavior on the substrate at elevated temperature. (a) Simulation of the shape of the 2.17 pL water droplet at the substrate at elevated temperature. (b) Magnified droplet–substrate interface and the shape of the vapor pocket beneath the droplet at various substrate temperatures. The full spectrum of the vapor pocket profiles is demonstrated in Figure S4 in the Supporting Information. (c) Calculated size of the vapor pocket at various temperatures. For temperatures below 100 °C (water boiling point), the pocket-size equation is unsolvable because of ΔT, which cannot be smaller than or equal to zero. Respective plots for IPA and TGME (Figure S1) are provided in the Supporting Information.

Figure 5

Flow velocity and direction within the water drop at various substrate temperatures. (a) At 25 °C, the flow is almost negligible and the middle vortex is located approximately 6 μm from the bottom of the drop while the side vortices are present 2.5 μm above the bottom of the drop. With increasing substrate temperature, the flow velocity is increasing, especially at the sides of the droplet. (b) At 130 °C, the velocity of the flow at the sides reaches 6 × 10^–3 m/s. Also, the central vortex is rising significantly while the side vortices move up and aside. For temperatures of (c) 220 and (d) 250 °C, the flow velocity reaches 12 × 10^–3 and 15 × 10^–3 m/s, respectively. In both cases, the central vortex forms approximately 15 μm from the bottom of the droplet. The side vortices occur 5.5 μm from the bottom of the drop and 5 μm to the side of the drop’s center.