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Publications from Group: 2012

Shedding light on helical microtubules: Real-time observations of microtubule self-assembly by light microscopy.
H. Y. Lee, H. Oh, J. H. Lee and S. R. Raghavan*
Journal of the American Chemical Society, 134, 14375 (2012)
The pathway by which vesicles transform into helical ribbons and then into closed tubules is visualized in real-time using light microscopy, The tubules are then shown to slowly rearrange to form larger plate-like structures.

A new approach for creating polymer hydrogels with regions of distinct chemical, mechanical, and optical properties.
S. J. Banik, N. J. Fernandes, P. C. Thomas and S. R. Raghavan*
Macromolecules, 45, 5712 (2012)
We create hybrid gels where different zones of the gel have distinct properties. Thereby, we hide a "message" written by one gel in the other - the message is revealed optically or thermally. This paper was featured by New Scientist.

The conundrum of gel formation by nanofibers, wormlike micelles, and proteins: Gelation without cross-links?
S. R. Raghavan* and J. F. Douglas*
Soft Matter, 8, 8539 (2012)
How do linear chains such as wormlike micelles and filamentous proteins form elastic gels even when cross-links are absent? We suggest that gelation can occur simply by physical entanglements if the chains are long and stiff.

Structural analysis of “flexible” liposome formulations: New insights into the skin-penetrating ability of soft nanostructures.
O. A. Ogunsola, S. Zhong, D. J. Pochan, R. Bronaugh and S. R. Raghavan*
Soft Matter, 8, 10226 (2012)
"Flexible" liposomes are believed to have the ability to squeeze through the stratum corneum, i.e., the outermost layer of skin. We show that these solutions are in fact mixtures of liposomes and micelles.

A new way for centrifugal separation of blood components: Creating a rigid barrier using a UV-curable thixotropic gel.
K. S. Sun, H. Oh, J. F. Emerson* and S. R. Raghavan*
Journal of Materials Chemistry, 22, 2378 (2012)
Blood is separated from serum by centrifugation with the help of a thixotropic gel that forms a squishy barrier between the two phases based on density differences. We show that by using a UV-curable gel, we can solidify this barrier into a hard solid., This paper was selected for the Back Cover by the journal and was featured in Chemistry World.

Microfluidic synthesis of monodisperse PDMS microbeads as discrete oxygen sensors.
K. Q. Jiang, P. C. Thomas, S. P. Forry, D. L. DeVoe* and S. R. Raghavan*
Soft Matter, 8, 923 (2012)
We use a microfluidic setup to create monodisperse microscale beads of PDMS (silicone). Moreover, we embed an oxygen-sensitive dye in these beads and show that the beads can sense the local oxygen concentration. .

Publications from Group: 2011

Light-activated ionic gelation of common biopolymers.
V. Javvaji, A. G. Baradwaj, G. F. Payne and S. R. Raghavan*
Langmuir, 27, 12591 (2011)
Biopolymers such as alginate can be gelled by Ca2+ ions. Such gels are used to host cells. We show that ionic gelation of alginate can be activated by UV light, using a commercial photoacid generator. This allows alginate to be gelled at precise locations. Our paper was selected for the cover of Langmuir.

A new approach to in situ “micro-manufacturing”: Microfluidic fabrication of magnetic and fluorescent chains of microparticles.
K. Q. Jiang, C. Xue, C. Arya, E. George, D. L. DeVoe* and S. R. Raghavan*
Small, 7, 2470 (2011)
We form microparticles of chitosan within a microfluidic chip and then we connect these particles on-chip into microchains. Magnetic and/or fluorescent chains of varying rigidity can thus be created (see movie). This paper was selected as a Frontispiece by the journal.

A self-assembling hydrophobically modified chitosan capable of reversible hemostatic action.
M. B. Dowling, R. Kumar, M. A. Keibler, J. R. Hess and S. R. Raghavan*
Biomaterials, 32, 3351 (2011)
We show that hydrophobically modified chitosan converts liquid blood into a gel (see movie) by insertion of hydrophobes into cell membranes. Gelation is reversed by addition of a-cyclodextrin. This work was featured in C&EN.

Reversible photorheological fluids based on spiropyran-doped reverse micelles.
H. Y. Lee, K. K. Diehn, K. Sun, T. Chen and S. R. Raghavan*
Journal of the American Chemical Society, 133, 8461 (2011)
We report a simple class of oil-based micellar fluids whose viscosity can be reversibly switched between low and high states by irradiation with light at different wavelengths. All components of the fluid are commercially available.

Nanoparticle-crosslinked hydrogels as efficient materials for separation and ion exchange.
P. C. Thomas, B. H. Cipriano and S. R. Raghavan*
Soft Matter, 7, 8192 (2011)
We show that hydrogels of NIPA crosslinked with laponite nanoparticles have a strong affinity for cationic solutes. In turn, they can selectively remove a cationic dye from a mixture of cationic and anionic dyes.

Biopolymer capsules bearing polydiacetylenic vesicles as colorimetric sensors of pH and temperature.
H. Y. Lee, K. R. Tiwari, and S. R. Raghavan*
Soft Matter, 7, 3273 (2011)
We create chitosan capsules having embedded vesicles of a diacetylenic amphiphile. These capsules are able to function as non-invasive colorimetric sensors of solution pH and temperature.

Vesicle capture on patterned surfaces coated with amphiphilic biopolymers.
M. B. Dowling, V. Javvaji, G. F. Payne* and S. R. Raghavan*
Soft Matter, 7, 1219 (2011)
We electrodeposit a hydrophobic derivative of chitosan on a gold surface and in a specific pattern. When this surface is exposed to vesicles, the vesicles are captured on the patterned regions via hydrophobic interactions.

Publications from Group: 2010

Can simple salts influence self-assembly in oil? Multivalent cations as efficient gelators of lecithin organosols.
H. Y. Lee, K. K. Diehn, S. W. Ko, S. H. Tung* and S. R. Raghavan*
Langmuir, 26, 13831 (2010)
We have found that salts can be dissolved in oil in the presence of a lipid and that the type of cation influences the assembly of the lipid. While Na+ has no effect, Ca2+ and La3+ induce cylindrical filaments and thus an organogel.

Thermogelling fluids containing low concentrations of Pluronic F127 and laponite nanoparticles.
K. Sun and S. R. Raghavan*
Langmuir, 26, 8015 (2010)
We show thermogelling, i.e., a conversion from liquid to gel upon heating, in mixtures of a triblock copolymer and clay nanoparticles. The effect is synergistic, i.e., it is shown by the mixture, but not the individual components.

Non-aqueous photorheological fluids based on light-responsive reverse wormlike micelles.
R. Kumar, A. M. Ketner and S. R. Raghavan*
Langmuir, 26, 5405 (2010)
We construct light-responsive non-aqueous fluids using a phospholipid and an organic derivative. The fluids undergo a 1000-fold drop in viscosity upon UV irradiation due to reductions in the lengths of reverse wormlike micelles.

Thermo-thickening in solutions of telechelic associating polymers and cyclodextrins.
R. Kumar and S. R. Raghavan*
Langmuir, 26, 56 (2010)
We show that miixtures of a water-soluble associating polymer and a sugar-based supramolecule, a-cyclodextrin exhibit thermo-thickening, i.e., an increase in viscosity upon heating.

Publications from Group: 2009

The distinct character of surfactant gels: A smooth progression from micelles to fibrillar networks.
S. R. Raghavan*
Langmuir, 25, 8382 (2009)
This Perspective argues that gel formation by surfactants is a process similar to micellization rather than crystallization; it is controlled by thermodynamics rather than kinetics.

Photogelling colloidal dispersions based on light-activated assembly of nanoparticles.
K. Sun, R. Kumar, D. E. Falvey and S. R. Raghavan*
Journal of the American Chemical Society, 131, 7135 (2009)
A low-viscosity sol of clay nanoparticles is shown to be converted to a gel (infinite viscosity) by UV irradiation. The key is the use of a photoacid generator, due to which the pH is lowered upon irradiation.

pH-responsive Jello: Gelatin gels containing fatty acid vesicles.
M. B. Dowling, J. H. Lee and S. R. Raghavan*
Langmuir, 25, 8519 (2009)
We entrap vesicles of oleic acid in gelatin gels (Jello). When the gel is placed in a high pH bath, the vesicles inside the gel are disrupted into micelles. This can be used to tune the rate of release of a dye from the vesicle-loaded gel.

Polymerizable vesicles based on a single-tailed fatty acid: A simple route to robust nano-containers.
J. H. Lee, D. Danino and S. R. Raghavan*
Langmuir, 25, 1566 (2009)
We show that polymerizable vesicles can be made using an inexpensive, commercially available fatty acid, UDA that has a terminal double bond. The polymerized vesicles can withstand the addition of detergents.

Persistence of birefringence in sheared wormlike micelles.
B. D. Frounfelker, G. C. Kalur, B. Cipriano, D. Danino and S. R. Raghavan*
Langmuir, 25, 167 (2009)
We describe and explain the unusual behavior of some wormlike micellar solutions whereby their flow-birefringence persists for hours and even days after shear is stopped. This behavior is shown by the movie here.

Photogelling fluids based on light-activated growth of zwitterionic wormlike micelles.
R. Kumar and S. R. Raghavan*
Soft Matter, 5, 797 (2009)
We describe a class of photogelling fluids based on a zwitterionic surfactant and an organic additive. The fluids undergo a 10,000-fold increase in viscosity upon UV irradiation due to the light-induced growth of wormlike micelles.

A simple method to improve the clarity and rheological properties of polymer/clay nanocomposites.
B. H. Cipriano, T. Kashiwagi, X. Zhang and S. R. Raghavan*
ACS Applied Materials & Interfaces, 1, 130 (2009)
Polymers are typically mixed with organoclays to make nanocomposites. We show that fractionating a commercial organoclay to remove large aggregates greatly improves the clarity and melt rheology of the nanocomposites.

Publications from Group: 2008

A facile route for creating “reverse” vesicles: Insights into “reverse” self-assembly in organic liquids.
S.-H. Tung, H.-Y. Lee and S. R. Raghavan*
Journal of the American Chemical Society, 130, 8813 (2008)
Reverse vesicles are self-assembled containers formed in nonpolar liquids (oils). We report a simple method to form reverse vesicles by mxing long- and short-chain lipids in conjunction with a small amount of salt.

Conductivity enhancement of carbon nanotube and nanofiber-based nanocomposites by annealing.
B. H. Cipriano, A. K. Kota, T. Kashiwagi, H. A. Bruck* and S. R. Raghavan*
Polymer, 49, 4846 (2008)
Conductivities of polymer/CNT and polymer/CNF composites can be very low after processing. We show that annealing the composites can restore the conductivity due to the re-formation of connections between the particles.

Strain-stiffening response in transient networks formed by reverse wormlike micelles.
S.-H. Tung and S. R. Raghavan*
Langmuir, 24, 8405 (2008)
Strain-stiffening, i.e., an increase in stiffness with deformation, is a property of many aqueous biopolymer gels. Here, we demonstrate the same unusual behavior for a viscoelastic solution of wormlike micelles in cyclohexane.

Self-assembled organogels obtained by adding minute concentrations of a bile salt to AOT reverse micelles.
S.-H. Tung, Y.-E. Huang and S. R. Raghavan*
Soft Matter, 4, 1086 (2008)
We show that adding millimolar amounts of the bile salt, SDC converts a dilute micellar solution of the lipid, AOT in cyclohexane into a stiff organogel. The gel shows biomimetic properties, including birefringence and strain-stiffening.

Liposome-templated supramolecular assembly of responsive alginate nanogels.
J. S. Hong, W. Vreeland, L. Locascio, M. Gaitan* and S. R. Raghavan*
Langmuir, 24,4092 (2008)
Gels of the biopolymer, alginate can easily be created at the macro or micro scales. Here we demonstrate a way to create nanoscale gels of alginate using liposomes as a template. The gels are studied by light scattering and TEM.

Publications from Group: 2007

A simple class of photorheological fluids: Surfactant solutions with viscosity tunable by light.
A. M. Ketner, R. Kumar, T. S. Davies, P. W. Elder and S. R. Raghavan*
Journal of the American Chemical Society, 129, 1553 (2007)
We report a simple class of surfactant fluids whose viscosity can be decreased 1000-fold by UV irradiation (see movie). The viscosity drop is due to the photoisomerization of an organic derivative, OMCA from trans to cis. This work was featured in Nature Materials, Materials Today, Separations Now, the NIST NCNR annual report for 2007, and a variety of other news sources.

Chitosan: A soft interconnect for hierarchical assembly of nanoscale components.
G. F. Payne* and S. R. Raghavan*
Soft Matter, 2, 521 (2007)
We highlight an 'Emerging Area', which is the use of the biopolymer, chitosan as a means to interconnect nanoscale components, such as nanoparticles, proteins, and vesicles. Our paper was featured on the inside cover of the issue.

Wormlike micelles of a C22-tailed zwitterionic betaine surfactant: From viscoelastic solutions to elastic gels.
R. Kumar, G. C. Kalur, L. Ziserman, D. Danino and S. R. Raghavan*
Langmuir, 23, 12849 (2007)
Can entangled wormlike micelles give rise to a true, elastic gel? Here, we show that this is indeed the case for worms formed by a long-tailed zwitterionic surfactant. Cryo-TEM, SANS and rheology are employed in this study.

Contrasting effects of temperature on the rheology of normal and reverse wormlike micelles.
S.-H. Tung, Y.-E. Huang and S. R. Raghavan*
Langmuir, 23, 372 (2007)
We show that the viscosity of reverse wormlike micelles in oil decreases more rapidly with temperature than that of normal wormlike micelles in water. This suggests that hydrogen-bonds are crucial for the growth of reverse worms.

Publications from Group: 2006

A new reverse wormlike micellar system: Mixtures of bile salt and lecithin in organic liquids.
S.-H. Tung, Y.-E. Huang and S. R. Raghavan*
Journal of the American Chemical Society, 128, 5751 (2006)
Bile salts are a class of natural surfactants with unusual properties. We show that adding small amounts of bile salts to a solution of the phospholipid, lecithin, in organic liquids, causes the growth of reverse wormlike micelles.

Self-assembly of surfactant vesicles that transform into viscoelastic wormlike micelles upon heating.
T. S. Davies, A. M. Ketner and S. R. Raghavan*
Journal of the American Chemical Society, 128, 6669 (2006)
We find that mixtures of the cationic surfactant CTAB with 5-methyl salicylic acid can form vesicles at room temperature. Upon heating, these vesicles transform into wormlike micelles, causing a 1000-fold increase in viscosity.

Transition from unilamellar to bilamellar vesicles induced by an amphiphilic biopolymer.
J. H. Lee, V. Agarwal, A. Bose, G. F. Payne and S. R. Raghavan*
Physical Review Letters, 92, 090898 (2006)
We show that the addition of a biopolymer with hydrophobic tails converts unilamellar vesicles into bilamellar (double-bilayered) ones. These results are confirmed by a combination of SANS and cryo-TEM.

Publications from Group: 2005

Vesicle-biopolymer gels: Networks of surfactant vesicles connected by associating biopolymers.
J. H. Lee, J. P. Gustin, T. Chen, G. F. Payne and S. R. Raghavan*
Langmuir, 21, 26 (2005)
We show that a biopolymer (chitosan) with hydrophobic tails can connect vesicles into a 3-D network, i.e., into a "vesicle gel". The vesicle gels are studied by rheology and SANS.

Viscosity increase with temperature in cationic surfactant solutions due to the growth of wormlike micelles.
G. C. Kalur, B. Frounfelker, B. Cipriano, A.Norman and S. R. Raghavan*
Langmuir, 21, 10998 (2005)
We show that cationic micelles in the presence of a naphthalene salt increase in length upon heating, causing the viscosity to rise. SANS and rheology complement each other in this study.

Salt effects on the phase behavior, structure and rheology of chromonic liquid crystals.
A. F. Kostko, B. Cipriano...M. A. Anisimov, D. Danino and S. R. Raghavan*
Journal of Physical Chemistry B, 109, 19126 (2005)
Cromolyn is an aromatic molecule that forms rodlike aggregates in water, which in turn order into nematic liquid crystals. We show that adding NaCl significantly widens the range of nematic behavior.

Anionic wormlike micellar fluids that display cloud points: Rheology and phase behavior.
G. C. Kalur and S. R. Raghavan*
Journal of Physical Chemistry B, 109, 8599 (2005)
We study wormlike micelles of sodium oleate in the presence of various salts. Triethyl amine salts are shown to induce cloud-point behavior, i.e., phase separation upon heating.


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