RESUMO
Molecular dynamics simulations in the microcanonical ensemble are performed to study the collapse of a bubble in liquid water using the single-site mW and the four-site TIP4P/2005 water models. To study system size effects, simulations for pure water systems are performed using periodically replicated simulation boxes with linear dimensions, L, ranging from 32 to 512 nm with the largest systems containing 8.7 × 106 and 4.5 × 109 molecules for the TIP4P/2005 and mW water models, respectively. The computationally more efficient mW water model allows us to reach converging behavior when the bubble dynamics results are plotted in reduced units, and the limiting behavior can be obtained through linear extrapolation in L-1. Qualitative differences are observed between simulations with the mW and TIP4P/2005 water models, but they can be explained by the models' differences in predicted viscosity and surface tension. Although bubble collapse occurs on time scales of only hundreds of picoseconds, the system sizes used here are sufficiently large to obtain bubble dynamics consistent with the Rayleigh-Plesset equation when using the models' thermophysical properties as input. For the conditions explored here, extreme heating of the interfacial water molecules near the time of collapse is observed for the larger mW water systems (but the model underpredicts the viscosity), whereas heating is less pronounced for the TIP4P/2005 water systems because its larger viscosity contribution slows the collapse dynamics. The presence of nitrogen within the bubble only starts to affect bubble dynamics near the very end of the initial collapse, leading to an incomplete collapse and strong rebound for the mW water model. Although nitrogen is non-condensable at 300 K, it becomes highly compressed and reaches a liquid-like density near the collapse point. We find that the dissolution of nitrogen is much slower than the movement of the collapsing water front, and the re-expansion of the dense nitrogen droplet gives rise to bubble rebound. The incompatibility of the collapse and dissolution time scales should be considered for continuum-scale modeling of bubble dynamics. We also confirm that the diffusion coefficient for dissolved nitrogen is insensitive to pressure as the liquid transitions from a compressed to a stretched state.
RESUMO
Using molecular dynamics simulations and transferable force fields, we designed a series of symmetric triblock amphiphiles (or high-χ block oligomers) comprising incompatible sugar-based (A) and hydrocarbon (B) blocks that can self-assemble into ordered nanostructures with sub-1 nm domains and full domain pitches as small as 1.2 nm. Depending on the chain length and block sequence, the ordered morphologies include lamellae, perforated lamellae, and hexagonally perforated lamellae. The self-assembly of these amphiphiles bears some similarities, but also some differences, to those formed by symmetric triblock polymers. In lamellae formed by ABA amphiphiles, the fraction of B blocks "bridging" adjacent polar domains is nearly unity, much higher than that found for symmetric triblock polymers, and the bridging molecules adopt elongated conformations. In contrast, "looping" conformations are prevalent for A blocks of BAB amphiphiles. Above the order-disorder transition temperature, the disordered states are locally well-segregated yet the B blocks of ABA amphiphiles are significantly less stretched than in the lamellar phases. Analysis of both hydrogen-bonded and nonpolar clusters reveals the bicontinuous nature of these network phases. This simulation study furnishes detailed insights into structure-property relationships for mesophase formation on the 1 nm length scale that will aid further miniaturization for numerous applications.