Microfluidic design DOI: 10.1002/smll.200600211 Flowing Lattices of Bubbles as Tunable, Self-Assembled Diffraction Gratings Michinao Hashimoto, Brian Mayers, Piotr Garstecki, and George M. Whitesides* W e demonstrate tunable, fluidic, two-dimensional diffraction gratings based on a microfluidic platform comprising a flow-focusing bubble generator and flowing, regular lattices of bubbles formed by dynamic self-assembly. The structure of these lattices can be tuned with switching times of less than ten seconds by changing the pressures and rates of flow applied to the device. These diffraction gratings exhibit high stability (over hours of operation if properly designed and operated). For our devices, we achieved tunable ranges in pitch from 12 to 51 mm, corre- sponding to first-order diffraction angles from 3.28 to 0.78 for light with a wavelength of 632 nm. Keywords: · bubbles · diffraction gratings · flow focusing · microfluidics · self-assembly 1. Introduction This paper demonstrates tunable and reconfigurable dif- fraction gratings composed of lattices of bubbles flowing in water containing a surfactant in microfluidic channels. Con- trol over the size and volume fraction of the bubbles is dem- onstrated to control the structure of flowing, regular lattices of bubbles that form by self-assembly. [1] These flowing latti- ces can be well ordered and stable, and their geometries and lattice constants can be tuned in real time with switch- ing times of less than ten seconds. Two types of microfluidic devices are demonstrated, which generate two-, and one-di- mensional diffraction patterns, respectively. The diffraction patterns are stable for extended periods of time (seconds to hours), with a pitch tunable from 12 to 51 mm. Here, the pitch denotes the periodicity of the self-assembled structure, determined by the apparent diameter of each bubble in the microfluidic channel. The corresponding range of the angle of the first-order diffraction peak for perpendicularly inci- dent light at l = 632 nm is from 3.28 to 0.78. Such tunable diffraction gratings can be used to 1) direct monochromatic light into a specified angle, and adjust this angle dynamical- ly, 2) sweep the angles of the diffracted beams at specified angular velocity, and 3) split multimodal light into separate, monochromatic beams, and change the color of the light dif- fracted at a particular angle dynamically. Optical components (diffraction gratings, lenses, and mirrors) are typically fabricated from rigid materials such as glass, quartz, rigid polymers, or metals. Specific applications, however, may require optics that can be modified structural- ly in real time, that is, “adaptive optics”. There have been several demonstrations of deformable solid optical compo- nents for the modulation of light. [2–8] Fluid-based optical components represent a class of components that is intrinsi- cally well suited for dynamic, reversible control over optical properties. [9–23] The adjustable parameters include both the geometry of the active components (e.g., the diameter of the core, in a liquid–liquid waveguide), and the property of the materials of which they are made (e.g., indices of refrac- tion of liquid, as controlled by the concentration of salts dis- [*] M. Hashimoto, Dr. B. Mayers, Prof. Dr. P. Garstecki, Prof. Dr. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University 12 Oxford St., Cambridge, MA (USA) Fax: (+ 1)617-495-9857 E-mail: gwhitesides@gmwgroup.harvard.edu Prof. Dr. P. Garstecki Institute of Physical Chemistry, Polish Academy of Sciences Kasprzaka 44/52, 01-224 Warsaw (Poland) Supporting information for this article is available on the WWW under http://www.small-journal.com or from the author. 1292 # 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2006, 2, No. 11, 1292 – 1298 full papers G. M. Whitesides et al.