Evaporative cooling and deposition patterns of evaporating $Al_2O_3$ nanofluid droplets

TL;DR

This study investigates how evaporative cooling influences deposition patterns of Al2O3 nanofluid droplets on hydrophobic surfaces, revealing temperature-dependent transitions from irregular networks to coffee-ring and dual-ring structures driven by thermocapillary flows.

Contribution

It introduces a non-dimensional parameter to characterize deposition pattern transitions and proposes scaling relations for evaporation flux, internal flow, and temperature profiles in evaporating nanofluid droplets.

Findings

Interconnected polygonal networks form at low temperatures (T_s ≤ 26°C).

Classical coffee-ring pattern appears at moderate temperatures (1 < Π_rel ≤ 10).

Dual-ring and central depositions occur at high temperatures (T_s > 40°C).

Abstract

The present study examines evaporative cooling and the resulting deposition patterns of a sessile $Al_2O_3$-based nanofluid droplet on a hydrophobic glass substrate at different temperatures. Evaporation predominantly occurs in the pinned contact line mode for both heated and non-heated cases, with only slight recession observed without heating. The droplet height and contact angle decrease linearly with time, and scaling relations are proposed to describe the evolution of droplet geometry and volume. A non-dimensional parameter, $\Pi_{rel}$, is introduced to characterize transitions in deposition patterns. For $\Pi_{rel} \leq 1$ ($T_s \leq 26^\circ$C), interconnected irregular polygonal network structures form at the periphery, which are rarely reported in evaporating droplets. With increasing substrate temperature, this structure is suppressed, giving rise to a classical coffee-ring pattern for $1 < \Pi_{rel} \leq 10$. At higher temperatures ($T_s > 40^\circ$C), dual-ring formation along with central particle deposition is observed for $\Pi_{rel} > 10$. The interfacial temperature is higher near the contact line and decreases toward the apex, and a universal scaling for the temperature profile is proposed. Internal flow velocity increases with substrate temperature, exhibiting asymmetric multi-vortex structures. Evaporative cooling intensifies with heating, enhancing evaporation flux and capillary flow. Appropriate scaling relations for evaporation flux and capillary velocity are established. Overall, the dynamics are governed by thermocapillary (Marangoni) flow induced by evaporative cooling, which enhances internal circulation and governs nanoparticle deposition morphology.