|
|
||
|
|
|
|
From the same paper as the previous 5 images.
Snapshots of the microinterferometry images during the deformation as insets in the force-deformation curve of a spherical microcapsule compressed as shown schematically in the previous slide. Labels A, B, C refer to deformation regimes: (A) no change in the contact area is noticed (pictures 1 and 2 are identical, as in [a] on the next slide); in (B) an increase in the adhesion area is observed, that is, an increase in the central dark zone in pictures 3-4 (as in [b] and perhaps in [c] on the next slide) and (C) a non-axisymmetric buckling of the spherical capsule in the adhesion area has occurred, that is, there are non-axisymmetric fringes inside the central area in pictures 5 and 6 (as in [d] on the next slide).
[The following paragraph was written by David Bushnell]
However, it seems possible, in view of what is pictured in [c] on the next slide, that the transition from Region A to Region B, where the force-deformation curve at first declines a very small amount and later in Region B becomes steeper than the original slope in Region A, represents axisymmetric buckling and post-buckling as depicted in [c] on the next slide; the rather dramatic decline in the force-deformation curve near the end of Region B represents a transition from axisymmetric post-buckling to non-axisymmetric bifurcation buckling and post-buckling with perhaps 2 or 3 circumferential waves; and the later rather mild decline in the force-deformation curve in Region C represents a transition from 3 to 4 circumferential waves in the post-buckled pattern.
Conclusions and outlook expressed in the paper from which this and the previous 5 slides are taken:
The mechanical properties of microcapsules are of obvious importance for practical applications, but they are also an intriguing object to study for physical reasons. Shells can be prepared with a high definition of layer thickness with layers of components drastically different in mechanical properties. It is this nanocomposite character which also lends abalone, muscle or bones their unique mechanical strength. It has become apparent that rather elegant and complementary techniques have become available to derive quantitative data on individual [micro-]shells. For future work it appears most promising to study the relation between modulus, adhesion and local curvature of shells as well as shape transitions. For the systems we have presented one expects that mechanics and adhesion can independently be varied which appears most promising also for applications. Also for polyelectrolyte multilayer capsules, we show that the laws of continuum mechanics hold down to dimensions of the order of 10 nm. Future trends in this area concern the limits of continuum mechanics, dynamic forces and the importance of structural transitions within individual layers.
Page 43 / 360