We study the deformation and dynamics of droplets in time-dependent flows using 3D numerical simulations of two immiscible fluids based on the lattice Boltzmann model (LBM). Analytical models are available in the literature, which assume the droplet shape to be an ellipsoid at all times (P.L. Maffettone, M. Minale, J. Non-Newton. Fluid Mech 78, 227 (1998); M. Minale, Rheol. Acta 47, 667 (2008)). Beyond the practical importance of using a mesoscale simulation to assess "ab-initio" the robustness and limitations of such theoretical models, our simulations are also key to discuss -in controlled situations -some relevant phenomenology related to the interplay between the flow time scales and the droplet time scales regarding the "transparency" transition for high enough shear frequencies for an external oscillating flow. This work may be regarded as a step forward to discuss extensions towards a novel DNS approach, describing the mesoscale physics of small droplets subjected to a generic hydrodynamical strain field, possibly mimicking the effect of a realistic turbulent flow on dilute droplet suspensions.

Charge separation at the donor–acceptor interface is a key step for high efficiency in organic solar cells. If interfacial hybrid states exist already in the dark it is plausible that they can have a major impact on the dissociation of optically generated excitations. In this work we probe such interfacial states via steady state absorption spectroscopy. A substantial bleaching of the absorption spectrum is found near the absorption edge when an electron‐accepting layer of either trinitrofluorenone (TNF), C60, or a perylene‐diimide derivative is deposited on top of a layer of electron‐donating conjugated polymers, such as MEH‐PPV or various poly‐phenylene. This is in part attributed to the formation of ground state complexes with low oscillator strength. The experiments bear out a correlation between the reduction of the absorbance with the energy gap between the donor‐HOMO and acceptor‐LUMO, the effective conjugation length of the donor, and the efficiency of exciton dissociation in the solar cell. The effect originates from mixing of the donor‐HOMO and the acceptor LUMO. Calculations using density functional theory support this reasoning. Implications for efficiency of organic solar cells will be discussed.

The deformation and dynamics of a single droplet in isotropic turbulence is studied using a Lattice Boltzmann diffuse interface model involving exact boundary flow conditions [1] to allow for the creation of an external turbulent flow. We focus on a small, sub-Kolmogorov droplet, whose scale is much smaller than the Kolmogorov length scale of the turbulent flow. The external flow field is obtained via pseudo-spectral simulation data describing the trajectory of a passive tracer in isotropic turbulence. In this way we combine the microscopic scale of the droplet and the macroscopic scale of the turbulent flow. The results obtained from this fully resolved model are compared to previous studies on sub-Kolmogorov droplet dynamics in isotropic turbulent flows [2], where an analytical model [3], which assumes the droplet shape to be an ellipsoid at all times, is used to describe the droplet deformation. Our findings confirm, that the hybrid pseudo-spectral Lattice Boltzmann algorithm is able to study droplet deformation and breakup in a regime well beyond the ellipsoidal approximation.