Tunable Nanoparticle Stripe Patterns at Inclined Surfaces

TL;DR

This work addresses programmable surface patterning by evaporative self-assembly of nanoparticles on inclined substrates, using gravity to drive stick-slip dynamics of the three-phase contact line. The method employs ∼2 μL AuNP suspensions on hydrophilic silicon with varying tilt angles $\phi$ and nanoparticle/surfactant concentrations to tune stripe density $\sigma$ and inter-stripe spacing $\lambda$. Key contributions include the first experimental demonstration of gravity-assisted semi-circular AuNP stripes formed via contact-line stick-slip, a reported $\sigma \sim \sin\phi$ relation, and tunability of $\lambda$ through interfacial tensions $\gamma_{LV}$ and $\gamma_{SL}$ by adjusting $C_{np}$ or surfactant. Overall, the results establish a programmable, gravity-interfacial-force framework for scalable nanoscale patterning with potential uses in biosensing, energy conversion, and nanofabrication.

Abstract

Periodic assemblies of nanoparticles are central to surface patterning, with applications in biosensing, energy conversion, and nanofabrication. Evaporation of colloidal droplets on substrates provides a simple yet effective route to achieve such assemblies. This work reports the first experimental demonstration of patterns formed through stick-slip dynamics of the three-phase contact line during evaporation of gold nanoparticle suspensions on inclined substrates. Variation in nanoparticle concentration and substrate inclination alter the balance of interfacial and gravitational forces, producing multiple stick-slip events that generate periodic stripes. Stripe density exhibits a sinusoidal dependence on inclination angle, while inter-stripe spacing remains nearly invariant. Independent control over inter-stripe spacing is achieved through adjustment of nanoparticle or surfactant concentration. These results highlight the complex interplay of gravitational and interfacial forces in directing periodic nanoparticle assembly and establish a versatile, programmable framework for surface patterning with tunable nano/microscale dimensions.

Figures (4)

• Figure 1: Transition from (a) sessile to (c) inclined droplet configuration resulting in a shift in the center of mass (c.m.). Forces acting on the TPCL, along with the particle flow velocities, are shown. Dried deposit resulting in a coffee-ring (b) for a sessile drop, and ordered stripe patterns after evaporation (d). A typical representation of AuNP deposition on a single stripe (rectangular box) is shown via a high-magnification SEM image in the inset of (d).
• Figure 2: FEG-SEM micrograph of dried AuNP droplet at $\phi=90^\circ$ (a) is shown here. Schematic (c) shows the fitting technique to obtain $\sigma,~\lambda$. Variation of stripe density $\sigma$ and stripe spacing ($\lambda$) with $\phi$ are plotted in (b,d), respectively. A representative line scan profile from the SEM image is shown in inset (d), where the peak-to-peak distances from the fitted plot are taken as the estimation of $\lambda$.
• Figure 3: Drop shape images recorded by optical tensiometer with varying gravity effect by changing the inclination angle $\phi$ (a-e). Dependence of $\theta_L,\theta_R$ and $\Delta\theta$ on $\phi$ are shown in (f).
• Figure 4: Ex situ dried patterns of $2.5,5,15~nM$ AuNP droplets at $\phi=120^\circ$ are shown in (a-c) [insets showing in situ droplet profiles at $t=0~s$]. Stripes start to form only for $C_{np}\geq 5~nM$. Stripe spacing $\sigma$ decreases with increasing $C_{np}$ (d), indicating enhanced pinning frequency. Consequently, with an increase in $C_{np}$, stripe density $\lambda$ increases, and the direct effect of $\lambda$ and $\gamma_{SL}$ is established for all the systems (e).