## 3 Results and Discussion
To study the environmentally responsive behavior of flat surfaces, sapphire discs were chosen and the particles which have been functionalized with ATRP initiator were adsorbed on the surface of these discs. The particles were then used to graft PNIPAAM brushes from the surface of the flat surfaces. It should be noted that a number of other sophisticated methods for the generation of responsive flat surfaces exist, but they are extremely cost-intensive owing to the requirement of laser etching and other similar processes. The process of adsorption of particles and subsequent grafting of polymer brushes is very cost-effective and does not require any complex processing of the surfaces. Figure 7 shows the process of adsorption of the latex particles on the flat substrates to make a uniform film once the water contained in the latex is evaporated. The generation of uniform film on the substrate is important as the uniform film is directly responsible for the uniform grafting of polymer brushes subsequently. If there is a lot of empty or dead space between the polymer particles, this space  
Figure 6: (a) Micrograph of the particles at 10 \({}^{\circ}\)C, (b) and (c) micrographs depicting the adsorption of protein and virus molecules respectively at 37 \({}^{\circ}\)C.  
would not contribute to any functional behavior of the surface. To generate a uniform film, heat treatment to the particles is generally provided so that they diffuse with each other optimally forming a thin layer. However, it is very important to optimize the heat treatment as too high a temperature or heating for too long may lead to complete melting of the polymer particles on the surface of discs and in this process, the ATRP initiator may be embedded deep into the melt and thus may not be accessible for the grafting reaction. Figure 8a shows the particles adsorbed on the positively charged sapphire discs at room temperature. By the generation of positive charges on the surface of discs, the negatively charged particles were observed to have much better adsorption on the surface of the discs as compared to the case of negatively charged discs. However, the surface coverage of the substrate with the particles was  
Figure 7: Process of film formation of polymer particles on a flat substrate and top view of the uniform film. still not optimum and the discs were heated at 100 \({}^{\circ}\)C and 150 \({}^{\circ}\)C for roughly 10 s. Figure 8b,c shows the particles after heat treatment at 100 \({}^{\circ}\)C. The coverage of the surface was observed to be much better as compared to room temperature. In Figure 8c, even two layers of particles were observed to be formed on the surface. The disc treated at 150 \({}^{\circ}\)C is shown in Figure 8d and it is clear that the temperature was too high as the particles were observed to be completely molten. Complete loss of identity of the particles may also affect the presence of ATRP polymer on the surface of the particles, thus affecting the grafting of PNIPAAM from the surface. Figure 9 shows the behavior of the PNIPAAM grafted sapphire discs (which were treated at 100 \({}^{\circ}\)C). At 10 \({}^{\circ}\)C, the droplet of water placed on the disc spread out quickly indicating the hydrophilic nature of the surface. On the other hand, when the disc was heated to 37 \({}^{\circ}\)C, the droplet of water did not spread out at all, indicating the change in the nature of the surface with temperature. The hydrophilic hydrophobic transition was also observed to be reversible thus confirming the successful translation of reversible behavior to the flat surface also. It should be noted that though the observed behavior is not superhydrophobic and superhydrophilic, the surfaces could be modified efficiently by  
Figure 8: (a) Late particles adsorbed on positively charged sapphire disc at room temperature, (b) and (c) adsorption of particles at 100 \({}^{\circ}\)C and (d) adsorption of particles at 150 \({}^{\circ}\)C.  
following a very simple and cost-effective technique.