Dynamics of droplet impingement on bioinspired surface: insights into spreading, anomalous stickiness and break-up

Abstract

Inspired by the self-cleaning ability of lotus leaves and stickiness (towards water) of rose petals, we investigate the droplet impact dynamics on such bioinspired substrates. Impact studies are carried out with water droplets for a range of impact velocities on glass, PDMS and soft lithographically fabricated replicas of the lotus leaf and rose petals, which exhibit near identical wetting properties as that of the original biological entities. In this work, we investigate the spreading, dewetting and droplet break-up mechanisms subsequent to impact. Surprisingly, the rose petal and lotus leaf replicas manifest similar impact dynamics. The observation is extremely intriguing and counterintuitive, as rose petal and its replicas are sticky in contrast to lotus leaves. However, these observations are based on experiments performed with sessile water droplets. By contrast, in the current study, we find that rose petal replicas exhibit non-sticky behaviour at the short time scale $\sim \left(O\right({10}^{-3}\left)\right)$ s similar to that exhibited by lotus leaf replicas. Air entrapment in the micrometre features of bioinspired surfaces prevent frictional dissipation of droplet kinetic energy, leading to contact edge recession. We have also unveiled interesting universal physics that govern the spreading, recession of the contact edge and subsequent break-up modes (ligament or bulb-ligament) of the droplet.

Keywords: bioinspired surfaces; droplet; droplet break-up; interface.

Figure 1.

(a) Scanning electron microcopy (SEM) images of original lotus leaf. Scale bar represents 15 µm. (b) SEM images of lotus leaf-inspired substrate. Scale bar represents 10 µm. (c) Atomic force microscopy (AFM) images of original rose petal. (d) AFM images of rose petal-inspired substrate. (Online version in colour.)

Figure 2.

Schematics of the experimental set-up. (1) DAQ system, (2, 3) high-speed camera (top and side view, respectively), (4) light source, (5) syringe pump, (6) platform, (7) substrate and (8) needle. (Online version in colour.)

Figure 3.

Notional diagram representing the hydrodynamic outcomes of droplet impact on different substrates for 6 < We < 132 (electronic supplementary material, videos S1 and S2). (Online version in colour.)

Figure 4.

Temporal variation of the spreading factor for different substrates: (a) glass, (b) PDMS, (c) lotus and (d) rose, where $\beta =d/d_0$ and $\beta =d_{max}/d_0$, d and d_max is droplet spread. The time scale is normalized with $t_{\text{harmonic}}\sim \sqrt{m\text{/}\sigma }$. (Online version in colour.)

Figure 5.

Droplet geometric attributes attained during spreading; instantaneous droplet spread (d), height (z) and contact angle θ. (a) Glass, θ < 90°. (b) PDMS, θ ∼ 90°. (c) Bioinspired substrates θ > 90°. (d) Variation of $\mathit{\Gamma }(\theta )$ with θ. (e) Contact angle at maximum spread θ as function of We. (f) Maximum droplet spread factor ${\beta }_{max}$ as function of $W{e}^{1/4}\mathit{\Gamma }{(\theta )}^{-1/2}$. (Online version in colour.)

Figure 6.

Temporal rate of change of the spreading factor; $\dot{\beta }$ for (a) glass, (b) PDMS, (c) lotus leaf replica, and (d) rose petal replica. The time scale is normalized with $t_{\text{harmonic}}\sim \sqrt{m/\sigma }$. The derivatives are calculated using the first-order forward difference Euler method (electronic supplementary material, figure S2). (Online version in colour.)

Figure 7.

Schematic of the jetting dynamics on PDMS substrate for We ∼ 6. (a) Collapse of the air-cavity ($R_{\text{cavity}}$) due to the influx of the vicinal fluid. (b) Formation of liquid jet which further undergoes tip break-up. Scale bar represents 1 mm (electronic supplementary material, video S2). (Online version in colour.)

Figure 8.

Temporal history of jet aspect ratio ($L_{\mathrm{jet}}\text{/}{w}_{\mathrm{jet}}$) for different We. For We ∼ 6, the jet satisfies the Rayleigh–Plateau criterion of break-up, $L_{\mathrm{jet}}\text{/}{w}_{\mathrm{jet}}>\pi$. Scale bar is 1 mm. (Online version in colour.)

Figure 9.

(a) Increase in air-cavity radius with initial impact velocity (experimental uncertainty is ±0.1%). (b) Dependence of Jetting velocity on $R_{\text{cavity}}$; theoretical estimation and experimental data points. (Online version in colour.)

Figure 10.

(a) AFM images of the rose petal replica. (b) Scanning electron microscopy (SEM) images of the lotus leaf replica; scale bar is 10 µm. (c) Schematic of the heterogeneous wetting for bioinspired substrates with non-uniform entrapped air-pockets. (Online version in colour.)

Figure 11.

Variation of receding velocity (V_rec) of the contact edge with maximum height attained by the droplet on bioinspired substrates; estimates from the scaling law and experimental values. (Online version in colour.)

Figure 12.

High-speed images depicting the bulb-ligament configuration at (a) We ∼ 6 and (b) We ∼ 16. The respective scale bars represent 2 mm. (c) The dependence of ligament height on the bulb diameter; theoretical estimate and the experimental data points. (Online version in colour.)

Figure 13.

(a) High-speed images of liquid-bulb pinch-off on the lotus substrates and forces responsible for the same (inset figure). (b) Snapshots of the liquid bulb on rose petal replicas, which do not exhibit pinch-off (electronic supplementary material, video S4). Scale bar represents 1 mm. (electronic supplementary material, video S4). (Online version in colour.)