Session E12: Drops: Sessile and Static Surface Interactions

Optimization of Aggregation Kinetics of SERS-active Nanoparticles in Evaporating Sessile Droplets

We studied evaporating sessile droplets containing silver nanoparticles as a platform for chemical detection using Surface-Enhanced Raman Spectroscopy (SERS), yielding molecule-specific Raman spectra. Controlling the degree of aggregation of nanoparticles is the key to the signal enhancement. Therefore, the aggregation kinetics of nanoparticles in droplets was investigated both experimentally and numerically to determine the evaporation and aggregation parameters leading to optimal detection. The signal depends on the degree of aggregation, which is affected by the initial concentration of nanoparticles (cNP), the dimerization rate (k), and the droplet evaporation time ($\tau $evap). We introduced the aggregation parameter $\Gamma $a $\equiv \tau $evap /$\tau $a, which is the ratio of the evaporation to the aggregation time scales. We found the different aggregation regimes based on aggregation parameter and stirring level within the droplet. For a well-stirred droplet, the optimal condition for SERS detection was found to be $\Gamma $a,opt $=$ k cNP $\tau $evap $\approx $ 0.3. The intensity of the SERS signal is near maximal in a wide range of aggregation parameters between 0.05 and 1.25 defining the time window during which trace analytes can be measured.   

Design of a Condensation-Based Contact Angle Goniometer

Condensation of low surface tension fluids such as refrigerants, natural gas, and carbon dioxide is important to a variety of industrial processes. Condensation of these fluids often occurs at elevated pressures and/or cryogenic temperatures, making measurement of their wetting properties using standard approaches challenging. It was recently demonstrated that these properties are critical in designing omniphobic surfaces for low surface tension fluid condensation rate enhancement [1]. To this end, we have developed an alternative goniometer design capable of contact angle measurement at wide pressure and temperature range. In this design, droplets are not dispensed through a pipette but generated through localized condensation on a tip of a preferentially cooled small metal wire encapsulated within a thick thermal insulator layer. Here we present a computational and an experimental study of the relation between the condensation-based goniometer geometry, subcooling, and droplet generation rate. We also compare water contact angle measurements using standard and condensation-based goniometer. \\[4pt] [1] Rykaczewski et al., Sci. Rep., 4, 2014.   

The bifurcation diagram of drops in a sphere/ plane geometry: influence of contact angle hysteresis

We study liquid drops that are present in a generic geometry, namely the gap in between a sphere and a plane. For the ideal system without contact angle hysteresis, the drop position is solely dependent on the contact angle, drop volume, and sphere/ plane separation distance. Performing a geometric analysis and Surface Evolver calculations, a continuous and fully reversible transition between axisymmetric non-spherical shapes and non-axisymmetric spherical shapes is predicted. We also study these transitions experimentally, varying the contact angle using electrowetting. Then, pinning forces drastically alter the pitchfork bifurcation as the unstable branch gets stabilized, and introduce a history-dependence in the system. As a consequence, the outward movement of drops following pinning can be either continuous or discontinuous, depending on the minimum contact angle that is attained.   

Hysteresis in the surface topography of laterally confined fluids

Possible steady states of a fluid within a vessel of finite width are theoretically studied. The fluid is under the action of surface tension, gravity and molecular interaction with the substrate. At steady state, the liquid surface can assume only two possible forms: a film of uniform thickness or a droplet surrounded by a uniform film. Depending on the volume of the liquid and the width of the containing vessel, the system will adopt one of the two steady states. However, there is a range of parameters for which both states can be reached either. In this case the system can present hysteresis, ie the final state may depend on its previous history.   

Asymptotic Expansion of the Axisymmetric Linear-Elastic Shell Equations with Application to Determining the Elastic Moduli of Ultra-Thin Shells

An asymptotic expansion of the axisymmetric linear-elastic shell equations is presented in the limit that h/r $\to $ 0 where h is the shell thickness and r is the characteristic radius of curvature. This solution is obtained using a WKB expansion, which doesn't rely on the shell being close to spherical, allows for turning points in curvature, and can be extended to include higher-order terms. The obtained solution is used to analyze experimental results for obtaining elastic moduli of ultra-thin shells of metal oxides on molten-metal drops.