Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R6: Focus Session: Evaporative Self-Assembly of Micro-and Nano-Particles |
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Chair: Daniel Attinger, Iowa State University Room: 309 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R6.00001: Drying of complex fluids at a moving contact line Ching Hsueh, Frederic Doumenc, Beatrice Guerrier We study the pattern formation induced by drying colloidal suspensions in a vertical Hele-Shaw cell immersed in a reservoir [1]. The contact line velocity can be well controlled by pumping out the solution from the reservoir. At low capillary number, we observe stick-slip motion and periodic strip deposition [2]. We measured the pinning force variation and the wavelength. We systematically vary the following parameters: receding velocity, evaporation rate, concentration, particle size, and pH of the suspension. Results allow determining the power law governing the pinning force variation. The pH, which has no effect on the pinning force variation, changes the deposition morphology significantly. Finally, we present a detailed comparison between colloidal suspensions and polymer solutions. \\[4pt] [1] H. Bodiguel, F. Doumenc, B. Guerrier EPJ-ST 166, 29-32 (2009) \\[0pt] [2] H. Bodiguel, F. Doumenc, B. Guerrier Langmuir, 13, 10758-10763 (2010) [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R6.00002: Dynamics of freely moving plates connected by a shallow liquid bridge Amir Gat, Homayun Navaz, Morteza Gharib We study the dynamics of freely moving solid bodies connected by a shallow liquid bridge via analytic and experimental methods. The gap between the solid bodies is used as a small parameter within a lubrication approximation, reducing the problem to an Abel equation of the second kind. Analysis of the governing differential equation yields two novel physical phenomena: (1) An impulse-like peak in the force applied by the liquid bridge on the solid bodies, obtained from a uniform asymptotic solution for small Capillary numbers. (2) Both linear and non-linear oscillations of the system for the case of surfaces with low wettability, obtained from small perturbations of the system around the equilibrium point. An experimental setup examining the motion of freely moving solid bodies was constructed, yielding experimental data which compared favorably with the analytic results and specifically displayed the predicted oscillations and impulse-like peak of the applied force. The application of the current analysis to the micro-manipulation of solid bodies and possible future research directions are discussed. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R6.00003: Real Time Electron Microscope Imaging of Nanoparticle Motion Induced by a Moving Contact Line Joseph Grogan, Haim Bau With the high resolution of the electron microscope, we imaged the interactions between receding and advancing contact lines and surface-bound nanoparticles. The experiments were carried out with a custom-made liquid cell, dubbed the nanoaquarium. The nanoaquarium seals a thin liquid layer between two thin, electron-transparent membranes, and allows one to image processes in liquid media with electrons. We observe that the nanoparticles are ejected in the wake of the receding contact line, and pushed by the advancing contact line. A simple mathematical model that accounts for surface tension and disjoining~pressure effects is constructed to interpret this curious phenomenon. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R6.00004: Field-directed assembly of nanoparticles Eric Furst The use of external fields to direct the assembly of colloidal suspensions, combined with new particle shape symmetries that couple strongly to such fields, is a powerful means for creating and tailoring materials with unique mechanical, optical and electronic properties [1]. I will present the evaporative assembly of nanostructured thin films from ellipsoidal titania nanoparticles. The deposition process is directed by an electric field. As the evaporation front recedes, a uniform film with thicknesses of 1-10 $\mu$m is deposited on the substrate. The films exhibit a large birefringence and high packing fraction due to the orientation of the particles. When the frequency is lowered, the particle orientation undergoes a parallel-random-perpendicular transition with respect to the field direction. The orientation dependence on field frequency and strength is explained by the polarizability of ellipsoidal particles. Particle orientation in the films also leads to anisotropic mechanical properties, which are manifested in their cracking patterns. In all, field-directed assembly of anisotropic particles provides a powerful means for tailoring nanoparticle film properties {\it in situ} during the deposition process. \\[4pt] [1] Grzelczak et al. Directed Self-Assembly of Nanoparticles. ACS Nano 4, 3591-3605 (2010). [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R6.00005: Suppressing the coffee stain effect: how to control colloidal self-assembly in evaporating drops using electrowetting Burak Eral, Dileep Mampallil Augustine, Michel Duits, Frieder Mugele We study the influence of electrowetting on the evaporative self-assembly and formation of undesired solute residues, so-called coffee stains, during the evaporation of a drop containing non-volatile solvents. Electrowetting is found to suppress coffee stains of both colloidal particles of various sizes and DNA solutions at alternating (AC) frequencies ranging from a few Hertz to a few tens of kHz. Two main effects are shown to contribute to the suppression: (i) the time-dependent electrostatic force prevents pinning of the three phase contact line and (ii) internal flow fields generated by AC electrowetting counteract the evaporation driven flux and thereby prevent the accumulation of solutes along the contact line Please see the link below for a short presentation and movies: http://www.youtube.com/watch?v=xwipCVZnN4E [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R6.00006: Vibration Assisted Convective Deposition Tanyakorn Muangnapoh, Alexander Weldon, Pisist Kumnorkaew, James Gilchrist A novel strategy for improving a convective deposition of aqueous binary suspensions of colloidal microspheres and nanoparticles was experimentally examined and reported. By adding a substrate vibration, an enhancement in deposited film qualities was observed. Moreover, by using this technique, it was easier and faster to obtain monolayer structures. In this experiment, we varied the amplitudes of substrate vibration between 0-330 $\mu $m. The quality of thin films was characterized by using a confocal laser scanning microscope and an image analysis. The motion of an interfacial liquid surface and the change in an evaporate rate due to a substrate vibration played an important role in an improvement of the deposition process. The monolayer structures formed from this rapid process can be used in a variety of optical, chemical, and biochemical sensing applications such as a LEDs device, a membrane separation and a cell capturing. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R6.00007: Interaction of bi-dispersed particles with contact line in inkjet-printed evaporating colloidal drops Viral Chhasatia, Ying Sun The deposition behavior of inkjet-printed aqueous colloidal mixture of micro and nanoparticles onto a glass substrate with systematically varied wettability has been investigated using fluorescence microscopy. Real-time bottom-view images show that particles inside an evaporating drop rearrange themselves near the drop contact line according to their sizes, where smaller particles tend to deposit closer to the contact line compared to the larger ones. By increasing substrate wettability, particles in the bi-dispersed mixture can be further separated compared to those on substrates of poor wettability. This is primarily because during different stages of evaporation, the interplay of surface tension, drag due to evaporative flow, and particle-substrate interactions, rearrange particles inside a colloidal drop near the contact line region. Forces acting on particles determine the extent to which particles enhance contact line pinning, which ultimately determines the final deposition morphology of particles from a bi-dispersed colloidal mixture. The effects of particle size contrast, particle volume fraction, and substrate surface energy on particle separation are examined in detail. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R6.00008: Fast Evaporation of Spreading Droplets of Colloidal Suspensions Kara Maki, Satish Kumar When a coffee droplet dries on a countertop, a dark ring of coffee solute is left behind, a phenomenon often referred to as ``the coffee-ring effect.'' A closely related yet less-well-explored phenomenon is the formation of a layer of particles, or skin, at the surface of the droplet. In this work, we explore the behavior of a mathematical model that can qualitatively describe both phenomena. We consider a thin axisymmetric droplet of a colloidal suspension on a horizontal substrate undergoing spreading and rapid evaporation. The lubrication approximation is applied to simplify the mass and momentum conservation equations, and the colloidal particles are allowed to influence droplet rheology through their effect on the viscosity. By describing the transport of the colloidal particles with the full convection-diffusion equation, we are able to capture depthwise gradients in particle concentration and thus describe skin formation, a feature neglected in prior models of droplet evaporation. Whereas capillarity creates a flow that drives particles to the contact line to produce a coffee-ring, Marangoni flows can compete with this and promote skin formation. Increases in viscosity due to particle concentration slow down droplet dynamics, and can lead to a significant reduction in the spreading rate. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R6.00009: Capillary-Driven Convective Assembly of Colloidal Monolayers Alexander Weldon, Pisist Kumnorkaew, Tanyakorn Muangnapoh, James Gilchrist Convective self-assembly is a powerful method for the deposition of particle thin films. We investigate the coupling between suspension properties and the deposition process during convective deposition of unary colloidal silica microspheres as well as the use of nanoparticles as packing aids. We can tune suspension and deposition properties to deposit submonolayer, monolayer, or multilayer morphologies. Thin films are analyzed via high speed confocal and scanning electron microscopy in order to generate local dynamic data describing the deposition process as well as the long-range structure of deposited thin films. [Preview Abstract] |
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