Chemistry Reference
In-Depth Information
Fig. 2.7
Time-resolved
micrograph of membrane
formation in a micro-fluidic
system.
To p
time-lapse images
of three-phase contact line
moving across the oil lamellae
while membrane is form-
ing. The total diameter of
the membrane is 300
m.
Bottom
Three-phase contact
line position as a function of
time. The slope of the
dotted
line
is 1.9mm/s
µ
constancy of the contact line velocity (i.e., the first derivative of the diameter with
respect to time) suggests a constant power of dissipation, which is due to the viscous
friction in the vicinity of the edge of the membrane, where the two aqueous phases
and the oil phase meet. More specifically, we can compare the contact line velocity
of about 1.9mm/s with the capillary velocity,
v
c
=
γ/η
=
is the
viscosity of the liquid (in this case Squalanewhich has a viscosity of 43.4mPa.s). This
is a natural velocity scale for the system, corresponding to the balance of interfacial
and viscous forces. Clearly, the measured contact line velocity is much less than
v
c
,
which can be attributed to the diverging viscous stress near the three-phase contact
line [
24
].
For an effective manipulation of the droplet configuration within the micro-fluidic
setup, it may be necessary to move the emulsion through the micro-fluidic device.
This can be done either externally by pressure or volume control (e.g., syringe
pumps), or internally by means of suitable local mechanisms. In any case, it is
of central importance that these manipulation steps do not lead to the destruction
of membranes. One might anticipate that this constraint poses a serious conceptual
4
.
1 cm/s , where
η
