Impact Desolvation of Polymers Embedded in Nanodroplets

Deposition patterns formed by the evaporation of linear diblock copolymer solution nanodroplets on solid surfaces

The evaporation-induced deposition pattern of the linear diblock copolymer solution has attracted attention in recent years. Given its critical applications, we study deposition patterns of the linear diblock copolymer solution nanodroplet on a solid surface (the wall) by molecular dynamics simulations. This study focuses on the influence of the nonbonded interaction strength, including the interaction between the wall and polymer blocks (ɛAW and ɛBW), the interaction between the solvent and the wall (ɛSW), and the interaction between polymer blocks (ɛAB). Conditions leading to diverse deposition patterns are explored, including the coffee-ring and the volcano-like structures. The formation of the coffee-ring structure is attributed to receding interfaces, the heterogeneity inside the droplet, and the self-assembly of polymer chains. This study contributes to the establishment of guidelines for designing deposition patterns of the linear diblock copolymer solution nanodroplet, which facilitates practical applications such as inkjet printing.

Splashing of Large Helium Nanodroplets upon Surface Collisions

In the present work we observe that helium nanodroplets colliding with surfaces can exhibit splashing in a way that is analogous to classical liquids. We use transmission electron microscopy and mass spectrometry to demonstrate that neutral and ionic dopants embedded in the droplets are efficiently backscattered in such events. High abundances of weakly bound He-tagged ions of both polarities indicate a gentle extraction mechanism of these ions from the droplets upon collision with a solid surface. This backscattering process is observed for dopant particles with masses up to 400 kilodaltons, indicating an unexpected mechanism that effectively lowers deposition rates of nanoparticles formed in helium droplets.

Desolvation of macromolecules by ultrafast heating: A molecular-dynamics study

Using molecular-dynamics simulation, we investigate the consequences of ultrafast laser-induced heating of a water droplet containing a solvated polymer, using the example of a 1 ps laser irradiation. We study the isolation process and the properties of the isolated polymer as a function of the polymer size, the droplet size, and the temperature to which the droplet is heated. We find that the isolation process occurs on a time scale of a few ten ps. The final polymer temperature increases linearly with the heating. Polymers embedded in larger droplets acquire higher temperatures, while larger polymers are less heated. In spite of the ultrafast heating, the isolated polymer remains in its coiled conformation.

Role of cohesive energy in droplet fragmentation

Using molecular-dynamics simulation, we investigate the fragmentation behavior of droplets after collision with a wall. We demonstrate that the ratio of the impact to the cohesive energy E(coh) of the droplet is the key quantity characterizing the droplet fragmentation process. To show this both van der Waals-bonded Ar and N(2) droplets and polar H(2)O droplets are studied. If the impact energy per molecule E<(0.35-0.4)E(coh), the droplet is reflected without fragmenting. Beyond that impact energy fragmentation of the droplet abruptly starts. At E=E(coh), the fragmentation process already results in a fine dispersal of the droplet into daughter droplets; the maximum fragment contains only less than 4% of the initial droplet mass and around one-third of the droplet has been shattered into isolated molecules. The disintegration process continuously increases with collision energy. These findings are relevant for the process of droplet fragmentation as used in the method of impact desolvation of electrosprayed microdroplets mass spectrometry.