Resonance induced wetting state transition of a ferrofluid droplet on superhydrophobic surfaces
Introduction
Many microfluidic applications utilize continuous-flow silicon, polymer, or glass-based microchannels equipped with valves, mixers, pumps, and sensors to perform rapid reactions, detection, and analyses [1], [2], [3]. While these devices have demonstrated significant promise, the integration of various components is often challenging and a bottleneck in lab-on-a-chip technologies. In addition, the adaptability of these devices to new applications or to the addition of new components is limited; typically, the devices have to be redesigned and newly fabricated, which can have significant lead times. More recently, discrete droplet-based approaches where a liquid plug is suspended in an inert carrier fluid have received interest Fair [4] owing to the ability to control individual droplets independently and to perform complex tasks in nano- to pico-liter volumes Niu et al. [5], Rastogi and Velev [6]. However, challenges due to contamination of isolated plugs by diffusion through the carrier fluid can occur and accurate volume conservation is difficult to achieve.
In contrast to closed channel microfluidic architectures, recent interests in controlling discrete liquid droplets on surfaces have emerged to help eliminate the need for microchannels. Mechanisms for droplet manipulation include electrowetting [4], [7], [8], [9], dielectrophoresis Gascoyne et al. [10], thermocapillarity [11], surface acoustic waves [12], [13], magnetic forces in combination with superparamagnetic particles [14], [15]. In particular, magnetic actuation has demonstrated several unique advantages [16]. An updated review on micro magnetofluidics can be found in Nguyen [17]. For example, superparamagnetic particles can be remotely manipulated by permanent magnets or electromagnets located off-chip, which provides the possibility to decouple the substrate in contact with the droplets from the actuation stage. In such a case, the surface can be low cost and disposable which eliminates cross-contamination, while the actuation stage (more expensive and complex) can be repeatedly used since it is not in direct contact with the fluid sample. In addition, the magnetic interaction is weakly dependent on pH, ionic strength, and temperature Pamme [16], which promises a more robust platform. Furthermore, magnetic actuation is particularly suitable for biological and biomedical applications since low frequency magnetic fields do not harm biological tissues Pamme [16]. However, in contrast to the other actuation mechanisms mentioned above, the magnetic force is a body force, i.e., the magnetic moment is proportional to the volume. As droplets decrease to microscale sizes, the actuation mechanism is less effective. For example, in the case of a droplet sliding on a surface, the smaller the droplet, the larger the frictional force is compared to the driving body force.
Combining the use of superhydrophobic surfaces with magnetic actuation, however, promise a platform to move, store, and mix microdroplets with low adhesion. In addition, by using periodic microstructured surfaces to create these superhydrophobic surfaces, to create superhydrophobic surfaces, the different wetting states, i.e., Wenzel or Cassie–Baxter state [18], [19], can be used to achieve three-dimensional manipulation. For example, the bottom surface can be functionalized, and upon wetting and dewetting of the droplet, a reaction can occur with the droplet and surface, which can be subsequently followed by mixing and droplet transport. While the Cassie to Wenzel transition by applying external fields and forces has been of particular fundamental interest to researchers [20], [21], the possibility of controlling wetting regimes with magnetic fields has not been demonstrated and offers new possibilities for the rapid development of microfluidic devices [22], [23], [24].
In this work, we induce Cassie to Wenzel wetting transitions of ferrofluid droplets on superhydrophobic microstructured surfaces using planar electromagnets. The wetting transition occurs by taking advantage of droplet resonance where an oscillating magnetic field is applied, and subsequently reduces the energy required to achieve the wetting transition. The demonstration of droplet wetting transition is an important component towards realizing a magnetic-based microfluidic platform in the future.
Section snippets
Experimental setup
Fig. 1 shows a schematic of the experimental setup used to manipulate and capture images of the ferrofluid droplet. The ferrofluid (EMG705, Ferrotec) used in the experiments, consists of magnetic particles with diameters of approximately 30 nm suspended in water, where the physical properties are reported in Table 1. The ferrofluid droplets ranging in volume from 1 μl to 10 μl were placed on the superhydrophobic microstructured surface (see Section 2.A) by pipette. The apparent contact angle,
Wetting transition
Fig. 4(a) shows time-lapse images with a droplet undergoing wetting transition at a frequency 44 Hz. In contrast, Fig. 4(b) shows a droplet undergoing magnetic induced oscillations at a frequency 5 Hz, different from resonant condition experiencing strong deformations but, no wetting transition. The wetting transitions occurred at frequencies associated with resonant conditions, as predicted by the following analysis. The resonant frequencies for various droplet sizes can be determined by using
Conclusions
In this paper we investigated an electromagnetic force to transition ferrofluid droplets on planar superhydrophobic surfaces. We showed that, even though the magnetic force is small at small liquid volumes, the superhydrophobic surface, characterized by low adhesion, provides the possibility to manipulate wetting transitions of droplets. We also showed that at the resonance, for the un-pinned contact line case, the deformation is so large that the Laplace pressure becomes large enough to induce
Acknowledgments
We wish to acknowledge R. Xiao for preparing the superhydrophobic surfaces and prof. M. Zahn and S. Khushrushahi for fruitful discussions and suggestions. Work done under the UniBSMIT-MechE faculty exchange Program co-sponsored by the CARIPLO Foundation, Italy under Grant No. 2008-2290.
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