Page 73 - Optofluidics Fundamentals, Devices, and Applications

54 Cha pte r T h ree

3-5-4 Bubble Grating
So far we have only focused on laminar systems using miscible flu-
ids. Another interesting system involves the use of immiscible fluids
to form emulsions and foams in microfluidic channels. These disper-
sions form periodic, self-assembled geometries that can also be recon-
figured in real time by varying fluid properties and flow conditions.
Here we describe a tunable, fluidic, two-dimensional diffraction grat-
ing based on a microfluidic system comprising a flow-focusing device
for generating bubbles, and a flowing, regular lattice of bubbles
formed by dynamic self-assembly [15].
As described in Chap. 2, the flow-focusing device consists of two
inlet channels for the liquid phase, and a single inlet channel for the
gaseous phase (Fig. 3-13a). The gas and liquid phases meet at a junc-
tion upstream of a narrow orifice. The gaseous thread periodically

Liquid
h
0.59 bar (0.91)
Gas Outlet
2 cm

Liquid
(a) (c) (e)
Camera

Rotating stage Objective 0.65 bar (0.91)

Mirror 2 cm
Sample
Screen 500 μm 200 μm
Light source (Inset)
(b) (d) (f)
FIGURE 3-13 Schematic representations of the experimental setup. (a) The fl ow-
focusing bubble generator and self-assembled lattice of bubbles. A tank of nitrogen
gas connects to the gas inlet, and digitally controlled syringes connect to the liquid
inlets. The system generates monodisperse bubbles that pack into a quasi-two-
dimensional sheet. (b) Diffraction and display of incident laser beam. Diffracted laser
light orthogonal to the plane of bubble lattice is displayed on a white screen. (c) and
(d) Packing of bubbles in a straight outlet channel that was 1-mm wide and 16-μm
high. The rate of fl ow of the continuous phase was 0.028 μL/s. The numbers denote
the input gas pressure and the volume fraction of bubbles (ϕ ) in parentheses.
vol
(c) 0.59 bar, ϕ = 0.91, and (d) 0.65 bar, ϕ = 0.91; their corresponding diffraction
vol vol
patterns are shown in (e) and (f), respectively. (M. Hashimoto, B. Mayers, P.
Garstecki, and G. M. Whitesides, “Flowing lattices of bubbles as tunable, self-
assembled diffraction gratings,” Small, 2, (2006), 1292–1298. Copyright Wiley-VCH
Verlag GmbH & Co. KGaA. Reproduced with permission.)