Capsules and microfluidics
Very few experimental data exist on the deformability of
initially-spherical artificial capsules. The mechanical properties of
capsules of millimetric size can be measured by compression under
parallel plates, micropipette aspiration or AFM. All these experimental
techniques can only be applied to a single capsule. We have developed a
new method of characterization in batch that can be applied to an
entire capsule population.
The membrane mechanical behavior of liquid filled microcapsules are
obtained using an inverse method. Cross-linked ovalbumin microcapsules
are flowed and deformed into a cylindrical microchannel of comparable
size. We measure simulataneously the deformation and speed of each
capsule through a rapid camera mounted on a microscope (figure 1a). The
capsule deformed shape is compared to predictions obtained numerically
when modeling a capsule under the same flow conditions (figure 1b). The
unknown shear modulus value corresponds to the best fit.
The degree of reticulation is estimated in parallel by determining the
free amino groups remaining on the microcapsules after the
cross-linking reaction. We characterize microcapsule populations
fabricated at different reaction pH (5–8) and times (5–30 min) to study
different cross-linking degrees.
The capsule shear modulus and the amino groups are nearly constant with
the reaction pH for the capsules fabricated after 5 min of
reticulation. The shear modulus increases with the reaction time, while
the NH2 content decreases with it. A global increase in shear modulus
with pH is also observed, together with an unexpected increase in NH2
content. The study shows that the inverse method is capable of
discriminating between various cross-linking degrees of microcapsules.
Moreover, for this type of microcapsules, the mechanical method appears
more reliable than the chemical one to obtain an estimation of their
cross-linking degree.
a. 
b. 
Figure 1: Example of 2 capsules, fabricated at pH 5 with tr = 5
min, flowing down the microchannel: a/R = 1, v = 1.4 mm/s (left), a/R =
0.95, v = 4.8 mm/s (right). (a) Pictures of the deformed capsule shape. (b) Corresponding superposition of the experimental (dotted line) and numerical (continuous line) profiles.
Collaborators
Prof. Dominique Barthès-Biesel, BMBI, UTC
Dr. Eric Leclerc, BMBI, UTC
Dr. Florence Edwards-Lévy, ICMR, Reims
Thi-Xuan Chu, Caractérisation des propriétés mécaniques de capsules par analyse inverse, PhD thesis, UTC, September 2011.
Benjamin Sévénié, Master thesis, UTC, September 2012.