RESUMEN
In the field of metallo-supramolecular assemblies, supramolecular macrocycles have attracted considerable attention due to their guest recognition and catalytic properties. Herein, we report a novel strategy for the construction of giant hollow macrocyclic structures using a bi-directional geometric constraint strategy. We investigated the structural design of two terpyridine-based tetratopic organic ligands, whose inner and outer rims have different angles. Compared to conventional strategies of self-assembly using single angular orientation building blocks that typically generate small macrocyclic objects or polymers, the mutual interaction between the different angles of the ligands could promote the formation of giant hollow macrocyclic supramolecular architectures. The self-assembly mechanism and hierarchical self-assembly of giant supramolecular macrocycles have been characterized by NMR, ESI-MS and TEM experiments. The strategy used in this study not only advances the design of giant 2D macrocycles with large inner diameters but also gives insights into the mechanism of formation of large structures.
RESUMEN
Droplet detachment from solid surfaces is an essential part of many industrial processes. Electrowetting is a versatile tool for handling droplets in digital microfluidics, not only on plain surface but also in 3-D manner. Here, we report for the first time droplet trampolining using electrowetting. With the information collected by the real-time capacitor sensing system, we are able to synchronize the actuation signal with the spreading of the droplet upon impacting. Since electrowetting is applied each time the droplet impacts the substrate and switched off during recoiling of the droplet, the droplet gains additional momentum upon each impact and is able to jump higher during successive detachment. We have modelled the droplet trampolining behavior with a periodically driven harmonic oscillator, and the experiments showed sound agreement with theoretical predictions. The findings from this study will offer valuable insights to applications that demands vertical transportation of the droplets between chips arranged in parallel, or detachment of droplets from solid surfaces.
RESUMEN
Aqueous sessile drops are launched from a super-hydrophobic surface by electric actuation in an electrowetting configuration with a voltage pulse of variable duration. We show that the jump height, i.e. the amount of energy that is transferred from surface energy to the translational degree of freedom, depends not only on the applied voltage but also in a periodic manner on the duration of the actuation pulse. Specifically, we find that the jump height for a pulse of optimized duration is almost twice as high as the one obtained upon turning off the voltage after equilibration of the drop under electrowetting. Representing the drop by a simple oscillator, we establish a relation between the eigenfrequency of the drop and the optimum actuation time required for most efficient energy conversion. From a general perspective, our experiments illustrate a generic concept how timed actuation in combination with inertia can enhance the flexibility and efficiency of drop manipulation operations.
RESUMEN
The effect of water concentration on the interfacial nanostructure and wetting behavior of a family of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2], at the surface of mica was investigated by contact angle measurement and atomic force microscopy (AFM). AFM reveals that interfacial layers of ILs observed at the surface of mica for "dry" ILs are not present for water-saturated ILs. The interaction of the IL ions of [Rmim][NTf2] with water molecules through hydrogen bonding is suspected to disrupt IL ion layering and precursor film growth on mica. Without the IL precursor film, contact angle relaxation of "wet" ILs on mica is less significant and ambient vapor adsorption becomes more important in determining the macroscopic wetting behavior.
RESUMEN
The connection between the interfacial properties of ionic liquids and their wetting behavior has been studied very little to date and not at all on heterogeneous surfaces. Therefore, we have investigated the static and dynamic wetting for a family of ionic liquids, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2], on mica, where R represents an ethyl, butyl, or hexyl alkyl chain on the imidazolium ring. Spreading is impacted greatly by a precursor film that forms on both homogeneous and heterogeneous mica surfaces. Macroscopically, the initial viscous spreading of the ionic liquid droplet on bare mica occurs within seconds but is then followed by a very slow relaxation that can be closely correlated with the typical time-scales of the precursor film growth. The contact angle for [emim][NTf2] and [bmim][NTf2] relaxes from about 40° to 23° over 30 and 90 min, respectively. For [hmim][NTf2], the process takes approximately 24 h and approaches complete wetting. The thickness of the precursor films for [emim][NTf2], [bmim][NTf2], and [hmim][NTf2] were 0.53, 0.65, and 1.0 nm, respectively, according to atomic force microscopy (AFM). These values are consistent with a monolayer of ionic liquid cations on mica, rather than ion pairs. A monolayer of octadecylphosphonic acid (OPA) on mica prevents both the formation of a precursor film and the relaxation of the contact angle. However, only a partial surface coverage of ~60% OPA is required to have the same effect. Quenching of precursor film formation (and associated contact angle relaxation) is due to an increasingly connected network of OPA regions that closes the nanoscale paths of bare mica on which the precursor film can develop via surface diffusion.