Jumping Nanodroplets

Abstract

Flat gold nanostructures on inert substrates like glass or graphite were illuminated by single intensive laser pulses with fluences above the gold melting threshold. The liquid structures produced in this way are far from their equilibrium shape, and a dewetting process sets in. On a time scale of a few nanoseconds, the liquid contracted toward a sphere. During this contraction, the center of mass moved upward, which could lead to detachment of droplets from the surface due to inertia. The resulting velocities were on the order of 10 meters per second for droplets with radii in the range of 100 nanometers.When small droplets impinge on a surface, varying degrees of deposition can be observed, ranging from sticking to rebounding. Sticking is essential for ink-jet printing and in agricultural agents that function by sticking to leaves; rebounding is desirable in cases such as selfcleaning surfaces (1, 2).The physics of impacting droplets has been well studied (3-5), and various types of (macroscopic) droplet sources have been developed (6). The impact-rebound process can be described energetically as the transformation of the impinging drop_s kinetic energy (KE) into surface deformation energy, followed by the inverse process, which detaches the drop (7).We examined whether it is possible to begin with deformed droplets on a surface and observe only the transformation from surface deformation energy to KE, as indicated by droplets jumping off the surface. For this purpose, we used droplets in the submicrometer range, which are much smaller than those typically used in impact studies. Such droplets can readily be obtained in an energetically unfavorable pancakelike shape with a large surface-to-volume ratio by preparing nanostructures in the solid state (e.g., by evaporation) on a substrate that in equilibrium is not wetted by the deposited material. Upon melting the nanostructures with a short laser pulse, dewetting sets in, and under appropriate conditions, detachment of the resulting droplets can be observed.The gold nanostructures we used here were fabricated by colloidal lithography, in which a monolayer of monodisperse spherical particles (with diameters of 1.5 to 3 mm) serves as a deposition mask (8, 9) to produce flat gold triangles with side lengths between 400 and 800 nm (Fig. 1A). The thickness of the evaporated films ranged between 50 and 160 nm. After removal of the colloid mask, these triangular gold structures were irradiated with a frequencydoubled Neodymium Doped Yttrium Aluminum Garnet (Nd:YAG)-laser (wavelength l 0 532 nm, full-width at half maximum of 10 ns). Because the absorption length of the laser radiation is smaller than the thickness of the nanostructure, we have to consider the temperature distribution inside of the nanostructure. An estimate for the thermal diffusion lengths on the time scale of the laser pulse yields 1600 nm for solid gold and 900 nm for molten gold, which is well above the thickness of the structures used here (10). Thus, we can assume that the temperature stays almos...

Optical near-fields of triangular nanostructures

“…We prepare such samples by illumination of the original triangular arrangement with a two-beam interference pattern of a nslaser. Reaching the melting temperature, the particles undergo a dewetting process which leads finally to the ejection of the particles from the surface [24]. Figure 8 depicts the result of such a preparation process after three annealing steps under different angles.…”

Real-Time Observation of Nanosecond Liquid-Phase Assembly of Nickel Nanoparticles via Pulsed-Laser Heating

“…Recently, pulsed-laser-induced dewetting of two-dimensional films 3-9 , one-dimensional lines and rings [10][11][12][13] , and lithographically patterned nanostructures 14,15 has demonstrated that understanding and controlling thin-film and Rayleigh-Plateau instabilities improves the ability to create organized metallic nanoparticle ensembles. Assembled particles have also been shown to "jump" or eject from one substrate and transfer to another depending on the energetics and dynamics of various laser-melted nanostructures [16][17][18] . Heretofore, the as described in the Methods section, the laser spot has a Gaussian profile and thus the radial profile of each image also contains thermal and temporal information.…”

Pulsed Laser Interference Patterning of Metallic Thin Films

“…These findings show the importance of inertia on the movement of liquids on the nanoscale not only for the annealing of nanostructures [10] but also for inhomogeneous illuminated thin films.…”