138
6 Active Gels
Fig. 6.29 Deformation of shells in the presence of a non-mesogenic dopant. Nematic alignment
is shown by black dashes and defects are enclosed by red circles. The color scale showing the
nematic order parameter is the same as in Fig. 6.28. (a) Elongated cylindrical surface; both sides
are shown. (b) Squat cylinder. The textures on the left and on the right in (a) and (b) correspond to
the positive and negative gradient interaction coefficients, respectively. (c) Spherical surface in the
Mercator projection. The isotropic domain is at the south pole (Zakharov and Pismen, 2017b)
defects are specific to a cylinder, and defects disappear if the cylinder is cut into a
square sheet, lacking the periodicity along its circumference.
With no dopant, the nematic alignment field on a sphere features four defects
with the charge +1/2 placed symmetrically at vertices of a tetrahedron. Rotation
by π around the isotropic domain reduces the required number of +1/2 defects to
just two, as shown in Fig. 6.29c, where the Mercator projection is used to make the
location of defects more evident.
6.8 Biomorphic Motion
Repeated restructuring may cause chunks of shape-changing polymers to move.
Most experimental studies use illumination to actuate geometrically active materials
dynamically. The effect of light commonly reduces just to heating the material,
thereby forcing a volume change or a transition from an ordered to the isotropic
state, but light can also cause structural changes, e.g., altering the gel’s hydrophilicity,
Fig. 6.30 (a) Artificial cilia bending due to isomerization caused by ultraviolet light (van Oosten
et al, 2009). (b) Artificial flagellum driven by the same mechanism. Scale bar 5mm (Huang et al,
2015). (c) Spiral-shaped relief patterns caused by isomerization in the polymer film illuminated by
a focused beam with helical wavefront. The side of the square is 6 μm. The elevation is indicated
by the shade changing from dark at −80 nm to light at 50 nm (Ambrosio et al, 2012)
6 Active Gels
Fig. 6.29 Deformation of shells in the presence of a non-mesogenic dopant. Nematic alignment
is shown by black dashes and defects are enclosed by red circles. The color scale showing the
nematic order parameter is the same as in Fig. 6.28. (a) Elongated cylindrical surface; both sides
are shown. (b) Squat cylinder. The textures on the left and on the right in (a) and (b) correspond to
the positive and negative gradient interaction coefficients, respectively. (c) Spherical surface in the
Mercator projection. The isotropic domain is at the south pole (Zakharov and Pismen, 2017b)
defects are specific to a cylinder, and defects disappear if the cylinder is cut into a
square sheet, lacking the periodicity along its circumference.
With no dopant, the nematic alignment field on a sphere features four defects
with the charge +1/2 placed symmetrically at vertices of a tetrahedron. Rotation
by π around the isotropic domain reduces the required number of +1/2 defects to
just two, as shown in Fig. 6.29c, where the Mercator projection is used to make the
location of defects more evident.
6.8 Biomorphic Motion
Repeated restructuring may cause chunks of shape-changing polymers to move.
Most experimental studies use illumination to actuate geometrically active materials
dynamically. The effect of light commonly reduces just to heating the material,
thereby forcing a volume change or a transition from an ordered to the isotropic
state, but light can also cause structural changes, e.g., altering the gel’s hydrophilicity,
Fig. 6.30 (a) Artificial cilia bending due to isomerization caused by ultraviolet light (van Oosten
et al, 2009). (b) Artificial flagellum driven by the same mechanism. Scale bar 5mm (Huang et al,
2015). (c) Spiral-shaped relief patterns caused by isomerization in the polymer film illuminated by
a focused beam with helical wavefront. The side of the square is 6 μm. The elevation is indicated
by the shade changing from dark at −80 nm to light at 50 nm (Ambrosio et al, 2012)