A review on control of droplet motion based on wettability modulation: principles, design strategies, recent progress, and applications

Abstract

The transport of liquid droplets plays an essential role in various applications. Modulating the wettability of the material surface is crucial in transporting droplets without external energy, adhesion loss, or intense controllability requirements. Although several studies have investigated droplet manipulation, its design principles have not been categorized considering the mechanical perspective. This review categorizes liquid droplet transport strategies based on wettability modulation into those involving (i) application of driving force to a droplet on non-sticking surfaces, (ii) formation of gradient surface chemistry/structure, and (iii) formation of anisotropic surface chemistry/structure. Accordingly, reported biological and artificial examples, cutting-edge applications, and future perspectives are summarized.

Fundamentals and Applications of Surface Wetting

“…Different strategies must be followed for various combinations of surface and external forces. 97 A proper technique must be chosen based on the requirement, properties of the liquid (surface tension and viscosity), and surface energies. A comparison (with DPFS) of achieved velocities through different passive transport processes is shown in Figure 4e.…”

“…However, in many applications, such as thermal management, chemical reactions, open surface microfluidics, energy harvesting, cellular studies, drug screening, oil–water separation, etc., it is required to control the motion of the liquid. Various strategies can achieve a controlled droplet motion: , Strategy 1: Droplet transport on a nonwettable surface due to a driving force Strategy 2: Droplet transport by creating chemical or structural gradient on the surface Strategy 3: Droplet transport due to geometry gradient …”

Liquid marbles: review of recent progress in physical properties, formation techniques, and lab-in-a-marble applications in microreactors and biosensors

“…The preparation of low-wettability particles is not challenging owing to the well-developed hydrophobic materials science (since the 1900s). 82,83 In early research, LMs were made using naturally or commercially available hydrophobic particles, such as lycopodium, PTFE, PVDF powder, carbon black soot, tonner particles, nanocarbons, fumed or colloidal silica nanoparticles, PS, PET, PDMS, wax particles, or long alkyl chain fatty acids. Some researchers focused on the formation of original hydrophobic particles (e.g., hydrophobically modified metals, metal oxides, MOFs, non-commercial polymers, and/or composite particles) to form functional LMs with properties that could not be obtained using commercially available particles.…”

Magnetically Actuated Surface Microstructures for Efficient Transport and Tunable Separation of Droplets and Solids

“…[ 8,19 ] Fluid transport can be realized by changing the adhesion state and surface tension between opposite sides of the droplet or bubble. [ 20,21 ] For solid object transport, a mechanical impulse is imparted by lamellar microstructures, propelling the object forward. [ 8,22 ] Alternatively, a localized dent arising on a compliant pillared surface in a nonuniform magnetic field [ 23 ] can also be used to guide the droplet or balls.…”