**Acknowledgements**

*21st Century Surface Science - a Handbook*

**2.6 Other stimuli**

**Figure 10.**

Some of the recent work also demonstrates using magnetic field and solvents as external stimuli to manipulate the wettability for given fluids. Smart surfaces that respond to magnetic field have been demonstrated by numerous research groups in the last decade [84, 85]. Grigoryev et al. fabricated a microstructured surface with reentrant geometry composed of Ni micronails, which shows a reversible transition from superomniphobic to omniphilic wetting state in response to the external magnetic field [85]. Cheng et al. also showed the reversible wetting transitions of the microdroplet consisting of superparamagnetic Fe3O4 nanoparticles [84]. They have shown that the wettability can be reversibly switched between the Cassie state and the Wenzel state on the microstructured silicon substrate and the transition between two wetting states can be controlled by both concentration of nanoparticles and the intensity of the magnetic field. On the other hand, solvent as an external stimulus is also feasible to tune the wetting behavior as solvent-responsive surfaces are affected by the surrounding medium and their wettability change is governed by the change in their interfacial energy caused by the rearrangement of the molecular chains in response to a solvent [81, 86–88]. Minko et al. demonstrated a novel route to fabricate two-level structured polymer brushes, and the surface wettability could be reversibly controlled by exposing the polymer brush surfaces to the solvent, which is selective to one of the polymers. Surface morphology and surface properties change when exposed to different solvents, which is caused by the interchange between vertical and linear phase segregation of the polymers [81].

*(A) Advancing and receding CAs of PEAALbL on a micro-patterned substrate with a square array of cylindrical pillar structures. Optical images show water droplets sticking to the substrate at pH 3, suggesting strong pinning. Inset in the upper right corner represents advancing and receding CAs of water droplets on the unpatterned substrate. (B–E) Cryo-SEM images of a water droplet on a PEAALbL-coated micropillar-*

*patterned substrate at (B, C) pH 3 and (D, E) pH 8. Reproduced with permission from [80].*

To conclude, smart surfaces with tunable wettability based on various external stimulus prove to be useful candidates in different applications where the continuous and reversible tuning of surface wettability is important. These surfaces exploit the additional energy gained from the external stimulus to change their wetting behavior as per the requirement. It has been demonstrated that the surface wettability can be varied from hydrophobic to hydrophilic on smooth surfaces and from

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**3. Conclusions**

The authors acknowledge fruitful discussion with Reeta Pant and Subhash Singha, which was particularly useful for the section "**Photoresponsive surfaces**".
