When a droplet gently lands on an atomically smooth substrate, it will most likely contact the underlying surface in about 0.1 s. However, theoretical estimation from fluid mechanics predicts a contact time of 10-100 s. What causes this large discrepancy, and how does nature speed up contact by 2 orders of magnitude? To probe this fundamental question, we prepare atomically smooth substrates by either coating a liquid film on glass or using a freshly cleaved mica surface, and visualize the droplet contact dynamics with 30-nm resolution. Interestingly, we discover two distinct speed-up approaches: (1) droplet skidding due to even minute perturbations breaks rotational symmetry and produces early contact at the thinnest gap location, and (2) for the unperturbed situation with rotational symmetry, a previously unnoticed boundary flow around only 0.1 mm=s expedites air drainage by over 1 order of magnitude. Together, these two mechanisms universally explain general contact phenomena on smooth substrates. The fundamental discoveries shed new light on contact and drainage research.

The impact and splash of liquid drops on solid substrates are ubiquitous in many important fields. However, previous studies have mainly focused on spherical drops while the non-spherical situations, such as raindrops, charged drops, oscillating drops, and drops affected by electromagnetic field, remain largely unexplored. Using ferrofluid, we realize various drop shapes and illustrate the fundamental role of shape in impact and splash. Experiments show that different drop shapes produce large variations in spreading dynamics, splash onset, and splash amount. However, underlying all these variations we discover universal mechanisms across various drop shapes: the impact dynamics is governed by the superellipse model, the splash onset is triggered by the Kelvin-Helmholtz instability, and the amount of splash is determined by the energy dissipation before liquid taking off. Our study generalizes the drop impact research beyond the spherical geometry, and reveals the potential of using drop shape to control impact and splash.

We have devised a simple method to determine the absorption and radiative decay rates of surface plasmon polaritons in an Au nanohole array by combining polarization-resolved reflectivity spectroscopy and temporal coupled-mode theory. The dependence of two rates on wavelength has been measured and they are found to agree with finite-difference time-domain simulations. As both absorption and radiative decay rates play a key role in several plasmonic applications, our approach offers a simple and effective means in determining them.

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Metamaterials are artificial materials that can achieve unusual properties through unique structures. In particular, their “invisibility” property has attracted enormous attention due to its little or negligible disturbance to the background field that avoids detection. This invisibility feature is not only useful for the optical field, but it is also important for any field manipulation that requires minimum disturbance to the background, such as the flow field manipulation inside the human body. There are several conventional invisible metamaterial designs: a cloak can isolate the influence between the internal and external fields, a concentrator can concentrate the external field to form an intensified internal field, and a rotator can rotate the internal field by a specific angle with respect to the external field. However, a multifunctional invisible device that can continuously tune across all these functions has never been realized due to its challenging requirements on material properties. Inside a porous medium flow, however, we overcome these challenges and realize such a multifunctional metamaterial. Our hydrodynamic device can manipulate both the magnitude and the direction of the internal flow and, at the same time, make negligible disturbance to the external flow. Thus, we integrate the functions of the cloak, concentrator, and rotator within one single hydrodynamic metamaterial, and such metamaterials may find potential applications in biomedical areas such as tissue engineering and drug release.

We have measured the radiative scattering from two-dimensional metallic arrays by using polarization-resolved reflectivity spectroscopy. We find the reflectivity spectra follow the Fano-like model that can be derived from temporal coupled mode theory and Jones matrix calculus. By orthogonally orienting the incident polarizer and the detection analyzer, reflectivity dips flip into peaks and the radiative scattering efficiency can be determined accordingly. The dependence of total radiative scattering efficiency on wavelength and hole diameter is found to agree well with Rayleigh scattering by single hole.

We measured the decay rates of two degenerate surface plasmon modes in Au nanohole arrays with different hole sizes by angle-resolved reflectivity spectroscopy. For each hole size, at the spectral region where resonant coupling occurs, we observed a large modification in decay rates, leading to the formation of dark and bright modes. The change in decay rates is well explained by temporal coupled mode theory. The deduced coupling constant is found to increase with increasing hole diameter. This study provides us a simple and effective means to control the decay rates of dark and bright modes, which are useful in plasmonic applications.