Tunable Thin Elasto-Drops

TL;DR

This work addresses creating centimeter-scale capsules that mimic liquid drops by decoupling surface tension from fluid properties via ultra-thin elastic shells. The authors fabricate centimetric elasto-drops using ball-assisted thinning of Ecoflex-based PDMS shells and characterize their mechanical response by exciting hydro-elastic waves on the shell. From the measured dispersion of surface waves, they extract the hoop tension and show a tension-dominated regime that behaves like an effective, tunable surface tension, enabling a direct analogy with drops. The study demonstrates that inflation and shell thickness can modulate the effective surface tension, making elasto-drops a robust model system for parametric studies of large-scale drops and informing design of tunable soft particles.

Abstract

We present an experimental method to fabricate centimetric thin elastic capsules with highly uniform thickness and negligible bending stiffness using silicone elastomers. In our experiments, the capsules thickness is tunable at fabrication, while internal pressure and hoop (circumferential) stress are adjustable via hydrostatic inflation once the capsules are filled and immersed in water. Capsules mechanics are probed through hydro-elastic waves generated by weak mechanical perturbations at the capsule interface. By analyzing the surface wave dynamics in the Fourier domain, we extract the in-plane stress and demonstrate that the hydro-elastic waves are exclusively governed by hoop stress. This establishes a direct analogy with liquid drops characterised by an effective surface tension, allowing the capsules to be modeled as large-scale "elasto-drops" with an inflation and thickness tunable effective surface tension. Our work demonstrates that elasto-drops serve as a robust model system for parametric studies of large-scale liquid drops with experimentally adjustable surface tension.

Figures (4)

• Figure 1: Fabrication process of the elasto-drop. a–e: Sequence of coating, draining, ball-assisted thinning, curing, demolding and liquid filling.
• Figure 2: a: Schematics of the hydro-elastic wave propagation on the surface of a pre-stretched elasto-drop. b: Example of the elasto-drop's surface edge detection, for an elasto-drop of initial thickness $h_0=125.5~\mathrm{\mu m}$, inflated at a strain $(R-R_0)/R=4~\%$, under a periodic forcing frequency $f_s=77.7$ Hz. The blue solid line represents the elasto-drop's detected edge. The zenith angle $\theta$ varying from 0 to $2\pi$ in the clockwise direction is defined with the vertical dashed line passing through the center and a solid line. c: Physical properties of the elastic shell and the surrounding fluids. The surface elevation is in the radial direction of the initially unperturbed sphere. d: Time variation of the surface elevation $\eta(\theta=0.82, t)$, obtained based on a classical edge detection by image intensity gradient. e: Zenithal variation of the shell's thickness.
• Figure 3: a,b: Instantaneous shapes of the elasto-drop surface under a-harmonic and b-impulse forcing, radially amplified by a factor of ten for better visibility. c,d: Spatiotemporal diagram of the surface amplitude $\eta$ as a function of the zenith angle $\theta$. The harmonic forcing frequency reads $f_s=77.7$ Hz. e: Dispersion relation $\omega(k)$ extracted from the frequency-wavenumber domain of the surface amplitude $\hat{\theta}(\omega, k)$ for harmonic (yellow) and impulse (blue). The dashed lines indicate the power-law fit $\omega=[T k^3/(2\rho)]^{1/2}$.
• Figure 4: a: Extracted tension of the elasto-drops, for increasing inflation, as a function of the in-plane strain. blue: $h_0=59.2~\mathrm{\mu m}$, violet: $125.5~\mathrm{\mu m}$ and orange: $185.5~\mathrm{\mu m}$. b: Tangential stress $\sigma_\theta$ as a function of the strain.