PANDA-D: Interface-free determination of two dynamic contact angles

Abstract

Contact angles are central descriptors of wetting and capillary behavior at solid--fluid interfaces. Dynamic contact angles in molecular simulations are commonly extracted from two-dimensional liquid--vapor contours and near-wall tangent fits. We introduce PANDA-D, an extension of Predicting Angle from Nanoscale Density Analysis (PANDA) that determines both contact angles of a driven liquid roll without extracting an interface. The density profile measures occupied layer width but does not identify which meniscus is advancing. The method resolves this ambiguity by jointly fitting density and the layerwise center-of-mass displacement relative to the roll. A circular-meniscus model maps these profiles to receding and advancing angles and an effective near-wall offset. On synthetic rolls with prescribed angles, the reconstruction error is $0.29^\circ$ root-mean-square. We then analyze force-driven Lennard--Jones rolls between crystalline walls. Joint fits reproduce both measured profiles, whereas density-only fits can match density while assigning incorrect asymmetry. The resulting angles agree with independently determined circle-fit angles to $0.84^\circ$ root-mean-square. Across four wall--fluid attractions, the receding angle decreases and the advancing angle increases with force, approximately linearly over the studied ranges. By eliminating contour thresholds and near-wall tangent extrapolation from the primary estimate, the two-profile approach enables reproducible, automated angle analysis across molecular trajectories.