Rebecca L. Agapov

Self-propelled sweeping removal of dropwise condensate

Dropwise condensation can be enhanced by superhydrophobic surfaces on which the condensate drops spontaneously jump upon coalescence. However, the self-propelled jumping in prior reports is mostly perpendicular to the substrate. Here, we propose a substrate design with regularly spaced micropillars. Coalescence on the sidewalls of the micropillars leads to self-propelled jumping in a direction nearly orthogonal to the pillars and therefore parallel to the substrate. This in-plane motion in turn produces sweeping removal of multiple neighboring drops. The spontaneous sweeping mechanism may greatly enhance dropwise condensation in a self-sustained manner.

Asymmetric Wettability of Nanostructures Directs Leidenfrost Droplets

Leidenfrost phenomena on nano- and microstructured surfaces are of great importance for increasing control over heat transfer in high power density systems utilizing boiling phenomena. They also provide an elegant means to direct droplet motion in a variety of recently emerging fluidic systems. Here, we report the fabrication and characterization of tilted nanopillar arrays (TNPAs) that exhibit directional Leidenfrost water droplets under dynamic conditions, namely on impact with Weber numbers ≥40 at T ≥ 325 °C. The directionality for these droplets is opposite to the direction previously exhibited by macro- and microscale Leidenfrost ratchets where movement against the tilt of the ratchet was observed. The batch fabrication of the TNPAs was achieved by glancing-angle anisotropic reactive ion etching of a thermally dewet platinum mask, with mean pillar diameters of 100 nm and heights of 200-500 nm. In contrast to previously implemented macro- and microscopic Leidenfrost ratchets, our TNPAs induce no preferential directional movement of Leidenfrost droplets under conditions approaching steady-state film boiling, suggesting that the observed droplet directionality is not a result of the widely accepted mechanism of asymmetric vapor flow. Using high-speed imaging, phase diagrams were constructed for the boiling behavior upon impact for droplets falling onto TNPAs, straight nanopillar arrays, and smooth silicon surfaces. The asymmetric impact and directional trajectory of droplets was exclusive to the TNPAs for impacts corresponding to the transition boiling regime, linking asymmetric surface wettability to preferential directionality of dynamic Leidenfrost droplets on nanostructured surfaces.

Lithography-free approach to highly efficient, scalable SERS substrates based on disordered clusters of disc on pillar structures

We present a lithography-free technological strategy that enables fabrication of large area substrates for surface-enhanced Raman spectroscopy (SERS) with excellent performance in the red to NIR spectral range. Our approach takes advantage of metal dewetting as a facile means to create stochastic arrays of circular patterns suitable for subsequent fabrication of plasmonic disc-on-pillar (DOP) structures using a combination of anisotropic reactive ion etching (RIE) and thin film deposition. Consistent with our previous studies of individual DOP structures, pillar height which, in turn, is defined by the RIE processing time, has a dramatic effect on the SERS performance of stochastic arrays of DOP structures. Our computational analysis of model DOP systems confirms the strong effect of the pillar height and also explains the broadband sensitivity of the implemented SERS substrates. Our Raman mapping data combined with SEM structural analysis of the substrates exposed to benzenethiol solutions indicates that clustering of shorter DOP structures and bundling of taller ones is a likely mechanism contributing to higher SERS activity. Nonetheless, bundled DOP structures appeared to be consistently less SERS-active than vertically aligned clusters of DOPs with optimized parameters. The latter are characterized by average SERS enhancement factors above 10(7).

Robust probes for high resolution chemical detection and imaging

There are needs for both high resolution imaging and high sensitivity detection/analysis of surface chemistry on a nanometer scale. These needs can be addressed with Raman spectroscopy coupled with schemes that provide extraordinary enhancement of the Raman signal, namely surface enhanced (SERS) and tip enhanced Raman spectroscopy (TERS). Advances in applications of high resolution imaging and high sensitivity detection will be enabled by two specific improvements: increased signal enhancement and increased robustness of the plasmonic structures needed to achieve enhancement. Robustness and stability are especially important for those plasmonic structures made of silver that usually provide the best enhancements. Here we focus particularly on TERS, in which a plasmonic structure is placed on a scanning probe microscope tip in order to achieve high lateral resolution imaging. We have demonstrated that aluminum oxide protected silver plasmonic structures show significantly increased robustness against chemical and mechanical degradation when compared to unprotected analogues without loss of enhancement. A 2-3 nm thick coating of aluminum oxide prevents chemical attack of the underlying silver film for three months in a desiccator, significantly increasing the storage life of current probes. The same protective coating also extends the scanning life of the probe when the probe is used to image a hard patterned silicon substrate.

Correction to Asymmetric Wettability of Nanostructures Directs Leidenfrost Droplets

Our further work with the high-speed camera revealed that a setting in the software (EPIXX-Cap LTDV3.7) caused only1outofevery4 frames tobesaved. Thiswasnotpreviously identified because the frame rate information contained in the saved .avi files corresponded to the expected capture rate of 1019 framesper second. The timebetween two successive frameswas used for the calculation of horizontal droplet velocities as well as the impact velocities used to calculate the Weber numbers. Therefore, all reported horizontal and impact velocities in the text should be divided by a factor of 4. All reported Weber numbers should be divided by 16 due to the fact that velocity is squared in the calculation. The time label in Figure 3b should have a time stampof 3.92ms rather than 0.98 ms. Figures 4 and 5 are shown below with corrected axes (velocity and/orWeber number). This correctiondoes not change any other aspects in the interpretation of data, analysis, or the articles conclusions. The authors apologize for this unintended systematic error.