Optimizing whiteness

Further optimizations are likely to come from reducing the volume/filling fraction of the colloidal films. We believe that the current films are limited by optical crowding as has been shown to be a critical feature in the evolutionary optimization of the Cyphocilus beetle shell.1 As such we have set out to fabricate anisotropic silica particles that pack down to lower densities upon drying. Larger, higher aspect ratio particles have the potential to exhibit interesting liquid crystalline phase behaviour20 or become trapped in transient

CHAPTER 5. ENGINEERING ANISOTROPIC SILICA COLLOIDS FOR BIO-INSPIRED WHITENESS OPTIMIZATION

states due to their increased sedimentation rate. These transient states could potentially involve the jamming of long ‘rod’-like particles akin to what was seen for PCC calcium carbonate in chapter 2 figure 2.11. The resultant colloidal films will have lower volume fraction with more voids between particles.

To test this hypothesis we synthesized silica rods with higher aspect ratio (l= 4.92 µm (0.16), d = 0.27 µm (0.31), l/d = 18), approximately double that of the previous ‘worm’-like sample. Indeed these high aspect ratio particles do apparently pack down to lower volume fractions particularly towards the top of the film (figure 5.10 b). We can approximate the volume fraction from the 2D projection of the SEM image (figure A.5) at the top of the film, as done with the previous samples, to giveφ= 0.51 which is approaching the optimum value determined for the Cyphocilus beetle scale _∼ 0.45 with similar fibril diameter to the rod diameters here.1 _{Interestingly the film appears to have two distinct} layers where particles have aligned near the bottom of the film to pack down densely, and towards the top of the film they are more isotropically distributed. A potential cause of this is likely convection whereby fluid flows towards the droplet edge driven by evaporation. The dense particles sediment near the bottom and get aligned with this flow. Some are swirled up and re-suspended and, as the film height slowly reduces from evaporation, the volume fraction increases and particles eventually jam into the less dense isotropic phase that is seen in the upper layers of the film. It is it not known whether this stratification of phases is beneficial for broadband light scattering.

Preliminary experiments have been undertaken to quantify the whiteness (lt) of these

new films. At the time of writing this thesis the analysis is still in progress, however we have measured the transmission for different film thickness (figure 5.11) and see a distinct reduction in transmitted light through the films in comparison to the previous lower aspect ratio colloidal films. These films clearly exhibit much higher broadband scattering with only a weak wavelength dependence across all films. The film with the lowest thickness in this series, 6.85 _± 0.81 µm, reflects around 80% of light hitting the sample despite being so thin. Such reflectance is on par with the Cyphocilus beetle scale. Quantifying the mean free path at λ = 600 nm, using the same procedure as for the previous samples, we find

CHAPTER 5. ENGINEERING ANISOTROPIC SILICA COLLOIDS FOR BIO-INSPIRED WHITENESS OPTIMIZATION

Figure 5.10. Typical assembled films of high aspect ratio (l/d = 18) ‘rod’-like particles. Scanning electron microscope (SEM) image of a cross-section of a film taken at a tilt angle of θ_∼40◦ _{a) and a high resolution image of the top of the film b). Interferometry surface}

image of a 10 _× 10 mm area of the film, transmission measurement area highlighted by dashed box with average film height of 17.31_±0.68µm. Scale bars: a) = 5µm, b) = 2µm and c) = 1 mm.

CHAPTER 5. ENGINEERING ANISOTROPIC SILICA COLLOIDS FOR BIO-INSPIRED WHITENESS OPTIMIZATION

T

� / nm

Figure 5.11. Total transmission,T, spectra across the visible spectrum for deposited high aspect ratio colloidal ‘rods’ films of different thicknesses ranging from _∼ 6 µm to 90 µm. Colour saturation represents thickness i.e. lighter colours = larger thickness.

materials with such low refractive index that perform this well in a whiteness application.

5.3

Conclusion

In this chapter we detailed an ongoing investigation into the use of anisotropic silica particles as scattering centres in assembled thin colloidal films. It is apparent from our preliminary results that the broadband reflection and thus whiteness is possible even in extremely thin films of low refractive index silica colloids. In fact by optimizing the geometry of the silica particles constituting the building blocks of the film, the performance rivals that of the Cyphocilus beetle – nature’s evolutionary optimized example. This an unprecedented result and shows promise for further investigation into the precise role of anisotropy in affording whiteness in colloidal films. From our findings we would suggest that anisotropy in the building blocks of the film is an essential parameter as it allows for intermediate volume fractions (φ_∼ 0.5) to be accessed whilst preserving disorder in the film necessary

CHAPTER 5. ENGINEERING ANISOTROPIC SILICA COLLOIDS FOR BIO-INSPIRED WHITENESS OPTIMIZATION

for diffuse scattering. We also suggest that optical crowding plays a role in the reduction of total optical scatter from most colloidal films and that a reduced packing fraction of particles can be achieved by using dense, higher aspect ratio particles.

5.4

Experimental

5.4.1 Materials

1-Pentanol (_≥99%), poly(vinyl-pyrrolidone) (PVP K-30: Average molecular weight = 40,000 g mol−1_{), tetraehtyl orthosilicate TEOS (98%) and sodium citrate tribasic dihydrate (99%)} were obtained from Sigma Aldrich. Ammonia (30%) and ethanol were obtained from Fischer Scientific.