@article{joshipura_nguyen_quinn_yang_morales_santiso_daeneke_truong_dickey_2023, title={An atomically smooth container: Can the native oxide promote supercooling of liquid gallium?}, volume={26}, ISSN={["2589-0042"]}, url={https://doi.org/10.1016/j.isci.2023.106493}, DOI={10.1016/j.isci.2023.106493}, abstractNote={Metals tend to supercool—that is, they freeze at temperatures below their melting points. In general, supercooling is less favorable when liquids are in contact with nucleation sites such as rough surfaces. Interestingly, bulk gallium (Ga) can significantly supercool, even when it is in contact with heterogeneous surfaces that could provide nucleation sites. We hypothesized that the native oxide on Ga provides an atomically smooth interface that prevents Ga from directly contacting surfaces, and thereby promotes supercooling. Although many metals form surface oxides, Ga is a convenient metal for studying supercooling because its melting point of 29.8°C is near room temperature. Using differential scanning calorimetry (DSC), we show that freezing of Ga with the oxide occurs at a lower temperature (−15.6 ± 3.5°C) than without the oxide (6.9 ± 2.0°C when the oxide is removed by HCl). We also demonstrate that the oxide enhances supercooling via macroscopic observations of freezing. These findings explain why Ga supercools and have implications for emerging applications of Ga that rely on it staying in the liquid state.}, number={4}, journal={ISCIENCE}, author={Joshipura, Ishan D. and Nguyen, Chung Kim and Quinn, Colette and Yang, Jiayi and Morales, Daniel H. and Santiso, Erik and Daeneke, Torben and Truong, Vi Khanh and Dickey, Michael D.}, year={2023}, month={Apr} }
 @article{morales_bharti_dickey_velev_2016, title={Bending of Responsive Hydrogel Sheets Guided by Field-Assembled Microparticle Endoskeleton Structures}, volume={12}, ISSN={["1613-6829"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84960373963&partnerID=MN8TOARS}, DOI={10.1002/smll.201600037}, abstractNote={Hydrogel composites that respond to stimuli can form the basis of new classes of biomimetic actuators and soft robotic components. Common latex microspheres can be assembled and patterned by AC electric fields within a soft thermoresponsive hydrogel. The field-oriented particle chains act as endoskeletal structures, which guide the macroscopic bending pattern of the actuators.}, number={17}, journal={SMALL}, publisher={Wiley}, author={Morales, Daniel and Bharti, Bhuvnesh and Dickey, Michael D. and Velev, Orlin D.}, year={2016}, month={May}, pages={2283–2290} }
 @article{morales_podolsky_mailen_shay_dickey_velev_2016, title={Ionoprinted Multi-Responsive Hydrogel Actuators}, volume={7}, ISSN={["2072-666X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84982797733&partnerID=MN8TOARS}, DOI={10.3390/mi7060098}, abstractNote={We report multi-responsive and double-folding bilayer hydrogel sheet actuators, whose directional bending response is tuned by modulating the solvent quality and temperature and where locally crosslinked regions, induced by ionoprinting, enable the actuators to invert their bending axis. The sheets are made multi-responsive by combining two stimuli responsive gels that incur opposing and complementary swelling and shrinking responses to the same stimulus. The lower critical solution temperature (LCST) can be tuned to specific temperatures depending on the EtOH concentration, enabling the actuators to change direction isothermally. Higher EtOH concentrations cause upper critical solution temperature (UCST) behavior in the poly(N-isopropylacrylamide) (pNIPAAm) gel networks, which can induce an amplifying effect during bilayer bending. External ionoprints reliably and repeatedly invert the gel bilayer bending axis between water and EtOH. Placing the ionoprint at the gel/gel interface can lead to opposite shape conformations, but with no clear trend in the bending behavior. We hypothesize that this is due to the ionoprint passing through the neutral axis of the bilayer during shrinking in hot water. Finally, we demonstrate the ability of the actuators to achieve shapes unique to the specific external conditions towards developing more responsive and adaptive soft actuator devices.}, number={6}, journal={MICROMACHINES}, publisher={MDPI AG}, author={Morales, Daniel and Podolsky, Igor and Mailen, Russell W. and Shay, Timothy and Dickey, Michael D. and Velev, Orlin D.}, year={2016}, month={Jun} }
 @article{morales_stoute_yu_aspnes_dickey_2016, title={Liquid gallium and the eutectic gallium indium (EGaIn) alloy: Dielectric functions from 1.24 to 3.1 eV by electrochemical reduction of surface oxides}, volume={109}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4961910}, DOI={10.1063/1.4961910}, abstractNote={Liquid metals based on gallium are promising materials for soft, stretchable, and shape reconfigurable electromagnetic devices. The behavior of these metals relates directly to the thicknesses of their surface oxide layers, which can be determined nondestructively by ellipsometry if their dielectric functions ε are known. This paper reports on the dielectric functions of liquid gallium and the eutectic gallium indium (EGaIn) alloy from 1.24 to 3.1 eV at room temperature, measured by spectroscopic ellipsometry. Overlayer-induced artifacts, a continuing problem in optical measurements of these highly reactive metals, are eliminated by applying an electrochemically reductive potential to the surface of the metal immersed in an electrolyte. This technique enables measurements at ambient conditions while avoiding the complications associated with removing overlayers in a vacuum environment. The dielectric responses of both metals are closely represented by the Drude model. The EGaIn data suggest that in the absence of an oxide the surface is In-enriched, consistent with the previous vacuum-based studies. Possible reasons for discrepancies with previous measurements are discussed.}, number={9}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Morales, Daniel and Stoute, Nicholas A. and Yu, Zhiyuan and Aspnes, David E. and Dickey, Michael D.}, year={2016}, month={Aug} }
 @article{morales_palleau_dickey_velev_2014, title={Electro-actuated hydrogel walkers with dual responsive legs}, volume={10}, ISSN={["1744-6848"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84893866937&partnerID=MN8TOARS}, DOI={10.1039/c3sm51921j}, abstractNote={Stimuli responsive polyelectrolyte hydrogels may be useful for soft robotics because of their ability to transform chemical energy into mechanical motion without the use of external mechanical input. Composed of soft and biocompatible materials, gel robots can easily bend and fold, interface and manipulate biological components and transport cargo in aqueous solutions. Electrical fields in aqueous solutions offer repeatable and controllable stimuli, which induce actuation by the re-distribution of ions in the system. Electrical fields applied to polyelectrolyte-doped gels submerged in ionic solution distribute the mobile ions asymmetrically to create osmotic pressure differences that swell and deform the gels. The sign of the fixed charges on the polyelectrolyte network determines the direction of bending, which we harness to control the motion of the gel legs in opposing directions as a response to electrical fields. We present and analyze a walking gel actuator comprised of cationic and anionic gel legs made of copolymer networks of acrylamide (AAm)/sodium acrylate (NaAc) and acrylamide/quaternized dimethylaminoethyl methacrylate (DMAEMA Q), respectively. The anionic and cationic legs were attached by electric field-promoted polyion complexation. We characterize the electro-actuated response of the sodium acrylate hydrogel as a function of charge density and external salt concentration. We demonstrate that "osmotically passive" fixed charges play an important role in controlling the bending magnitude of the gel networks. The gel walkers achieve unidirectional motion on flat elastomer substrates and exemplify a simple way to move and manipulate soft matter devices and robots in aqueous solutions.}, number={9}, journal={SOFT MATTER}, publisher={Royal Society of Chemistry (RSC)}, author={Morales, Daniel and Palleau, Etienne and Dickey, Michael D. and Velev, Orlin D.}, year={2014}, pages={1337–1348} }
 @article{palleau_morales_dickey_velev_2013, title={Reversible patterning and actuation of hydrogels by electrically assisted ionoprinting}, volume={4}, ISSN={["2041-1723"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84881440081&partnerID=MN8TOARS}, DOI={10.1038/ncomms3257}, abstractNote={The ability to pattern, structure, re-shape and actuate hydrogels is important for biomimetics, soft robotics, cell scaffolding and biomaterials. Here we introduce an 'ionoprinting' technique with the capability to topographically structure and actuate hydrated gels in two and three dimensions by locally patterning ions via their directed injection and complexation, assisted by electric fields. The ionic binding changes the local mechanical properties of the gel to induce relief patterns and, in some cases, evokes localized stress large enough to cause rapid folding. These ionoprinted patterns are stable for months, yet the ionoprinting process is fully reversible by immersing the gel in a chelator. The mechanically patterned hydrogels exhibit programmable temporal and spatial shape transitions, and serve as a basis for a new class of soft actuators that can gently manipulate objects both in air and in liquid solutions.}, number={1}, journal={NATURE COMMUNICATIONS}, publisher={Springer Nature}, author={Palleau, Etienne and Morales, Daniel and Dickey, Michael D. and Velev, Orlin D.}, year={2013}, month={Aug} }