Chemistry Reference
In-Depth Information
6.3 Results
6.3.1 Marangoni Stresses Propel Droplets
We demonstrate this squirmer scheme with nanoliter droplets containing 25mM
bromine water in a continuous oil phase of squalane containing 50mM mono-olein
(MO). The critical micelle concentration (CMC) is 1.5mM. The droplets are confined
by two hydrophobic glass plates to a quasi 2 dimensional space, thus simplifying
droplet tracking. The top panel of Fig.
6.3
shows the trajectory of a single squirmer
droplet over a duration of 400 s. The velocity of the droplet in this duration is roughly
constant at about 15
sec. The trajectory is reminiscent of a random walk, with a
persistence length which is larger than the droplet size and clearly far beyond what
would be expected for Brownian motion. A particularly important observation is
that the trajectory crosses itself. This is seen even more convincingly in the multiple
µ
m
/
Fig. 6.3
To p
schematic of a micro-droplet squirmer. Bromination increases the tension of the
droplet surface from 1.8 to 3.1mN/m. The convective flow pattern (shown in the rest frame of the
droplet) is accompanied by a gradient in the bromination density. The corresponding Marangoni
stress propels the droplet.
Middle
(
left
) path of a single squirmer droplet. The persistence length
is clearly large compared to the droplet radius, indicating propelled motion (Scale bar: 300
µ
).
Middle
(
right
): Droplet trajectories over an interval of
90 s.
Bottom
time lapse series of seven
droplets in a microchannel. The droplets change direction without noticeable reduction in velocity
∼
