Chemical Engineering Science 55 (2000) 3335}3340 Dynamic deformation visualization in swelling of polymer gels Evdokia C. Achilleos, Robert K. Prud'homme, Kostas N. Christodoulou, Kyle R. Gee, Ioannis G. Kevrekidis* Department of Chemical Engineering, Princeton University, School of Engineering and Applied Science, Olden Street, Princeton, NJ 08544-5263, USA Avery Research Center, Pasadena, CA, USA Molecular Probes Inc., Eugene, OR, USA Received 14 May 1999; accepted 20 December 1999 Abstract A technique for the real-time visualization of transparent gel deformation is developed, based on caged photo-activated #uorophores covalently attached to the gel network. The technique is used to quantitatively monitor the swelling of quasi two-dimensional sodium polyacrylate gels in a constrained geometry.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Deformation visualization; Material line tagging; Concentration "eld; Gel swelling 1. Introduction Polymer gels are an integral part of technologies ranging from diapers (superabsorbent gels) (Buchholz & Peppas, 1994) and photographic "lms (gelatin) (Christodoulou, Kistler & Schunk, 1997), to in situ drug delivery (Park, 1997), arti"cial muscles, actuators and sensors (Shiga, 1997; Lee, 1996; Shahinpoor, 1996). Final material properties crucially depend on processing his- tory (transport, stress development); the quantitative and resolved measurement of gel deformation during swelling and drying is therefore vital in understanding and event- ually optimally designing the materials. Here we report the development of an experimental technique for the spatially resolved real-time visualization of deformation pro"les in transparent polymer gels. The technique is based on potentially #uorescent, caged water-soluble photo-activated #uorophores (PAFs), attached to a monomer (in our case, acrylamide). Copolymerization of this compound with monomers results in gels with the dye permanently anchored on the polymer network. Un- caging the #uorophores with a laser beam `writes a linea in this gel; we wrote material grids and visualized their movement as the gel deformed while swelling. * Corresponding author. Tel.: 001-609-258-2828; fax: 001-609-258- 0211. E-mail address: yannis@arnold.princeton.edu (I.G. Kevrekidis) The technique originates from photo activated nonin- trusive tracking of molecular motion (PHANTOMM), a #ow tagging velocimetry technique (Lempert, Magge, Ronney, Gee & Haugland, Lempert et al., 1994). In PHANTOMM trace concentrations of PAFs are dis- solved in the #ow liquid and a pulsed UV laser beam is used to cleave (uncage) the caging group (which acts as a #uorescense quencher when attached to the PAF). This e!ectively writes a line in the #ow, and velocity measure- ments are performed by following the temporal displace- ment of this line when it is illuminated by a second light source that stimulates #uorescence. The anchoring of the dye to the polymer network (by copolymerizing dye- tagged monomers) is an essential component of our tech- nique; un-anchored dye would have di!used through the gel during the long times associated with a gel swelling experiment. Another motivation for this work was our computational transient modeling of the swelling and drying of polymer gels in constrained geometries. In these (and comparable) simulations extensive data on the transient deformation, concentration and stress "elds are produced (Christodoulou, Lightfoot & Powell, 1998; Powell, Christodoulou & Kevrekidis, 2000). The experi- mental availability of concentration and deformation data would provide a more detailed test of transport models. Alternate techniques to measure di!usion and swelling in/of gels have included tagging free polymer molecules with a #uorescent dye to study di!usion coe$cients 0009-2509/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 0 0 2 - 6