Evaporation of squeezed water droplets between two parallel hydrophobic/superhydrophobic surfaces.

HYPOTHESIS: A liquid droplet is apt to be deformed within a compact space in various applications. The morphological change of a droplet and vapor accumulation in the confined space between two parallel surfaces with different gaps and surface wettability are expected to significantly affect the evaporation dynamics of the squeezed droplet therein. EXPERIMENTS: Here the evaporation dynamics of a squeezed droplet between two parallel hydrophobic/superhydrophobic surfaces are experimentally explored. By reducing the surface gap from 1000 µm to 400 µm, the evolution of contact angle, contact radius and volume of the evaporating droplet are measured. A diffusion-driven model based on a two-parameter ellipsoidal segment geometry is developed to predict the morphology and volume evolution of a squeezed droplet during evaporation. FINDINGS: Evaporation dynamics of a squeezed water droplet via the constant contact radius (CCR) mode, the constant contact angle (CCA) mode, or the mixed mode are experimentally observed. Confirmed by our ellipsoidal segment model, the evaporation of the squeezed droplet is significantly depressed with the decreasing surface gap, which is primarily attributed to vapor enrichment in a more confined geometry. A linear scaling law between droplet volume and evaporation time is unveiled, which is verified by a simplified cylindrical model.

Flexible approach to vibrational sum-frequency generation using shaped near-infrared light.

We describe a new approach that expands the utility of vibrational sum-frequency generation (vSFG) spectroscopy using shaped near-infrared (NIR) laser pulses. We demonstrate that arbitrary pulse shapes can be specified to match experimental requirements without the need for changes to the optical alignment. In this way, narrowband NIR pulses as long as 5.75 ps are readily generated, with a spectral resolution of about 2.5 cm-1, an improvement of approximately a factor of 3 compared to a typical vSFG system. Moreover, the utility of having complete control over the NIR pulse characteristics is demonstrated through nonresonant background suppression from a metallic substrate by generating an etalon waveform in the pulse shaper. The flexibility afforded by switching between arbitrary NIR waveforms at the sample position with the same instrument geometry expands the type of samples that can be studied without extensive modifications to existing apparatuses or large investments in specialty optics.