Low-Weber-Number Droplet Splitting and Retention on Circular Hydrophilic Patterns Embedded in a Superhydrophobic Surface

Localized wettability patterning provides a passive strategy for regulating droplet impact, rebound, and retention without external energy input. In this study, the impact dynamics of deionized-water droplets on circular hydrophilic patterns embedded in a superhydrophobic surface were experimentally investigated using high-speed imaging. The Weber number (We = ρdv2/σ, representing the ratio of inertial force to surface-tension force) was varied from 2 to 150, and three dimensionless hydrophilic-pattern diameters, D* = 0.4, 0.8, and 1.2, were considered. The results show that the hydrophilic pattern has a limited influence on the early inertia-dominated spreading stage but plays a decisive role during recoiling and rebound through contact-line pinning. Depending on We and D*, three characteristic impact regimes are identified: adhesion, partial rebound, and splashing. Notably, for a circular hydrophilic pattern with a diameter of 1 mm (D* = 0.4), partial rebound occurs at We = 3.5. During partial rebound, the lower portion of the droplet remains pinned within the hydrophilic region, whereas the upper portion detaches through liquid-bridge breakup, enabling passive generation of secondary droplets. The normalized retained volume V* increases markedly with D* but exhibits a nonmonotonic dependence on We, reflecting the competition between enhanced pinning and inertia-driven fragmentation. An empirical correlation, V* = a0+a1D* + a2We + a3(D*)2 + a4D*We + cWe2, is proposed to quantify the retained liquid volume within the partial-rebound regime. Contact-line analysis further reveals that the maximum outer-rim diameter and the maximum three-phase contact-line diameter both increase approximately linearly with We, whereas the difference between them gradually decreases as inertia increases. These findings provide quantitative guidance for passive droplet capture, controllable generation of millimeter- and submillimeter-scale droplets, and the design of wettability-patterned surfaces for microfluidic and thermal-management applications.