Methods of droplet transport have future applications in areas ranging from micro-cooling systems to biological assays. Previous studies have shown that drops of liquid can be made to move across several types of surface gradients, including chemical and roughness gradients. Surface gradients have the disadvantage of being inherently finite in length and thus, limit the distance a drop can travel. This limitation can be overcome by employing the use of textured, super hydrophobic surfaces. It has been shown that when given a vertical excitation (vibrated on top a speaker), droplets can move across the textured surfaces. These surfaces, or ratchets, are of specified geometry and roughness. Varying ratchet geometry has the effect of predictably controlling droplet motion and to some extent, its speed. The main purpose of this research is to characterize the vertical excitation parameters (frequency and acceleration) in terms of both effectiveness and efficiency and to analyze the dynamic properties of a droplet in motion. Furthermore, the effect of varying input parameters, such as droplet volume and type of liquid was also explored. It has been shown that effective excitation parameters are dependent upon ratchet geometry and droplet volume. In general, ratchets with lower roughness values (phi) have lower ranges of frequency that can induce movement. Additionally, larger drops tend to have larger ranges of effective frequency. Liquids with high surface energy, meaning strong adhesion and weak cohesion, tend to spread out and cannot move at all; thus, water, which has low surface energy, is an ideal liquid for this type of system.

We report on a novel digital microfluidic device capable of achieving droplet transport using vertical vibration and curved hydrophilic rungs patterned onto a hydrophobic surface. The original Surface Ratchets were implemented using microscopically rough features on polydimethylsiloxane (PDMS) or silicon with a hydrophobic coating. Flat Surface Ratchets take advantage of periodic asymmetric wetting barriers by patterning the wettability of a substrate with self-assembled monolayers (SAMs). We report two novel device fabrications, a hydrophilic trimethylsilanol SAM patterned onto either a hydrophobic perfluorooctyltrichlorosilane (FOTS) or dodecanethiol SAM. Transport of a 10 μl droplet is demonstrated by applying vertical sinusoidal vibrations with an electromagnetic speaker with amplitudes low as 37 μm at 82 Hz. The ease of Flat Surface Ratchet fabrication, a simple wettability pattern, makes the technology implementable on a massive range of substrates opening up numerous potential applications. For example, the trimethylsilanol-FOTS Flat Surface Ratchet is an optically flat device, yielding a fully transparent device if fabricated onto glass. Such a device could be easily integrated onto a car’s windshield, and droplet transport could potentially be driven by the natural vibrations of a car.

We report on a novel digital microfluidic device capable of achieving droplet transport using vertical vibration and curved hydrophilic rungs patterned onto a hydrophobic surface. The original Surface Ratchets were implemented using microscopically rough features on polydimethylsiloxane (PDMS) or silicon with a hydrophobic coating. Flat Surface Ratchets take advantage of periodic asymmetric wetting barriers by patterning the wettability of a substrate with self-assembled monolayers (SAMs). We report two novel device fabrications, a hydrophilic trimethylsilanol SAM patterned onto either a hydrophobic perfluorooctyltrichlorosilane (FOTS) or dodecanethiol SAM. Transport of a 10 μl droplet is demonstrated by applying vertical sinusoidal vibrations with an electromagnetic speaker with amplitudes low as 37 μm at 82 Hz. The ease of Flat Surface Ratchet fabrication, a simple wettability pattern, makes the technology implementable on a massive range of substrates opening up numerous potential applications. For example, the trimethylsilanol-FOTS Flat Surface Ratchet is an optically flat device, yielding a fully transparent device if fabricated onto glass. Such a device could be easily integrated onto a car’s windshield, and droplet transport could potentially be driven by the natural vibrations of a car.