The manipulation of microscopic droplets has garnered significant interest. The exploration of physical, chemical, and biological functionalities of microfluidic devices on laboratory platforms offers rapid and cost-effective methods for biochemical analysis, droplet reactions, and microscale detection. Liquid transport can be achieved via surface energy gradients, facilitating droplet movement over short distances. Other approaches, such as thermal driving, electrowetting, magnetic driving, and optical driving, have specific limitations. For instance, thermal drive methods employing directional propulsion surfaces with asymmetric microstructures in the Leidenfrost state lead to a significant local temperature increase, which is unfavorable for droplet-based microreactions and chemical analyses. Although electrowetting achieves droplet motion by inducing large contact angle changes using electric fields, it is frequently constrained by dielectric breakdown. Magnetic driving requires doping magnetic particles into droplets or substrates, which compromises biological detection and chemical analysis. Similarly, light-driven approaches rely on light-induced surface tension differences, permitting the transport of only tiny droplets. Mechanical vibration offers a simple and efficient strategy for droplet manipulation, with unique advantages. Unlike magnetic driving, it avoids cross-contamination as it does not require adding specific particles to the droplets. By altering the micro-design of the surface, mechanical vibration can effectively transport droplets. However, the application of vertical mechanical vibration for transporting biocompatible droplets on patterned surfaces and facilitating droplet microreactions and their detection has not been reported.

In this study, we employed femtosecond laser direct writing (FLDW) and lubricating oil injection to fabricate a smooth circular arc groove array (SCGA). FLDW technology was selected for its high resolution, efficiency, speed, and exceptional patterning capabilities. Circular arc micro-slot arrays were created on polydimethylsiloxane (PDMS) substrates. The synergistic effects of PDMS’s intrinsic hydrophobicity and the micro-nano roughness induced by laser processing rendered the surface superhydrophobic, enabling the subsequent injection of lubricating oil to form a tightly adherent oil film. To achieve a smooth and chemically homogeneous silicone oil film over the textured substrate, an excess volume of silicone oil lubricant was impregnated into the SCGA. However, directional droplet motion necessitates anisotropy. Therefore, revealing the asymmetric properties of the arc array was critical. For this, spinning was employed to remove excess lubricant by optimizing rotational speed and duration, producing the final SCGA with the desired properties.

The prepared SCGA exhibits excellent hydrophobicity, with a water droplet contact angle of approximately 120°. Although this value is below the superhydrophobic standard of 150°, due to the inherent characteristics of the lubricating oil, the SCGA demonstrates extremely low contact angle hysteresis. Directional droplet transport was achieved under vertical vibration actuation [Fig. 1(b)]. The SCGA was fixed onto a vibration generator platform tilted at 0.9° to eliminate random motion of the horizontal surface. A function signal generator produced sinusoidal signals to control the vertical vibration of the platform. When parameters such as frequency and amplitude were optimized, a 10 μL droplet began moving along the y-axis, while elongating along the z-axis. The droplet moved a distance of 279 mm in 27.9 s, corresponding to an average speed of 10 mm/s [Fig. 1(b)]. The motion of the droplet, driven by vertical vibration, is attributed to the synergistic effects of the driving force generated by vibration and the anisotropic resistance induced by the asymmetric SCGA [Fig. 2(a)]. The droplet's motion involves two distinct states: wetting and dewetting. Dewetting is generally more sensitive to the microtextures of the substrate compared to wetting, resulting in isotropic spreading and anisotropic contraction of the three-phase contact line. Over a single vibration period, the droplet advances by a few structural periods, with cumulative droplet advancement leading to net transport over time [Fig. 2(b)]. At a vibration amplitude of V_pp=20 V , the speed of water movement increased to 21.5 mm/s as the vibration frequency rose from 31 to 83 Hz. At a fixed vibration frequency of 45 Hz, the speed of water movement reached 27.67 mm/s as the amplitude increased from V_pp=9 V to V_pp=20 V [Figs. 3(c),(d)]. To investigate potential applications of this study, droplet microreaction experiments were performed using the SCGA platform under vertical vibration (Fig. 4).

Directional droplet transport is critical in numerous applications, including biochemical microanalysis, microfluidic devices, and water mist collection. Among various driving mechanisms such as mechanical, thermal, electric, magnetic, and optical driving, mechanical vibration offers distinct advantages, including pollution-free operation and excellent biocompatibility. In this study, a SCGA was developed by combining FLDW with lubricating oil injection. The platform successfully demonstrated the directional transport of droplets with varying surface tensions and enabled droplet microchemical reactions. These findings highlight the potential of the proposed strategy for biomedical applications and droplet manipulation.