Research

Patchy Particles   •    Particles at Interfaces   •    Field Assembly   •    Dye-sensitized Solar Cells   •    Electroactive Polymers   •    Swimmers   •    Metamaterials   •    Particle Films   •    Surface Modification   •    Three-dimensional Assembly   •    Particles as Templates


Evaporation of Particle-laden Droplets

Using molecular dynamics simulations (MD) to study the evaporation of particle-laden droplets on a heated surface, trying to investigate the contact line pinning phenomena with Janus particles, which are modeled as nano-size, rigid balls with two distinguish sides. Various statistical variables(i.e.: temperature, evaporative flux) are measured so as to be compared with theories and experiments, as seen in Figure 1.

Figure 1. MD simulation of drople evaporation: density profile (left), temperature contour (middle) and velocity distribution (right).

The evaporation process (see Figure 2) and the deposition patterns of the particles are the most concernd things in this research, we test the validity of some of the underpinnings of the theoretical analyses used in the problem. We also research this self-assembly problem by varying the interactions between the wall-liquid, liquid-particle and particle-particle. The simulations reveal the connection between particle interactions and deposit structure, and indicate some limitations in continuum modeling for nanodrops. The significance of this research is that simple numerical simulation offer a viable method for both testing the theoretical underpinnings of the process and for predicting the nature of the outcome – the structure of the resulting particle deposit.

Figure 2. Three sequential snapshots of a evaporation process from a typical simulation.



Students:
Current Graduate Students: Weikang Chen
Collaborators: Prof. J. Koplik

Posters:

Relevant Literature:
[1] W. Chen, J. Koplik and I. Kretzschmar, to be published

Funding:
• This work was supported in part by NSF-CBET (CAREER) 0644789


Two-dimensional Nano- and Microparticle Films

This research area is concerned with the development of simple methods for the fabrication of highly ordered, close-packed particle monolayers. Our objectives are the reproducible formation of monolayer areas and the minimization of defects within the monolayers. Particle monolayers are intersting precursors for:
• chemical and pressure sensors
• particle modification technology
• shadow masks.

We have developed a simple cell set-up which allows the control of the miniscus of the drying suspension (Fig. 1a).[1] The set-up enables us to create a concave miniscus, which leads to the fromation of a two-dimensional particle film in the center of the cell as the particle suspension dries (Fig. 1b).


Within the two-dimensional particle film, immersion capillary forces[2] are at work which cause attraction between particles (Fig. 2a) leading to close packing of the particles. In addition, evaporation occurs in the area of the monolayer particle film. As a result, solvent needs to be replenished in the monolayer region by a convective flow from the cell wall towards the cell center. Particles are transported by the convective flow towards the 2D-particle film in the center of the cell leading to the growth of the monolayer (Fig. 2b).


In our studies, we find that the particle volume fraction and the evaporation rate are the determining factors in the convective assembly of particles. We are able to form large area monolayers of ~45 mm 2using the above described set-up. The monolayers show packing defects (grain boundaries) due to small size of the domains. Best monolayer crystals are obtained with: f = 0.002, tdrying ~ 90 min, and a cell load of 25 ml (see Poster).

Students:
Current Graduate Students: Amar B. Pawar
Current Undergraduate Students: Anton Dela Cruz, Mohammad Mahmud, Ralph Eddy Daniel
Former Contributors: Carlos Alberto Silvera Batista, Charlotte König, Saumya Banerjee

Posters:
1)“Assembly of Two-dimensional Monolayers from Micro- and Nanoscale Particle Dispersions” C. A. Silvera Batista and I. Kretzschmar* Columbia NSEC Retreat, Spring 2006 PDF

Relevant Literature:
[1]“Two-dimensional Micro- and Nanoparticle Monolayer Films” C. A. Silvera Batista and I. Kretzschmar* Proceedings of the Junior Scientist Conference '06, 2006. PDF
[2]“Capillary forces and structuring in layers of colloid particles” P. A. Kralchevsky and N. D. Denkov Current Opinion in Colloid & Interface Sci., 2001. 6 383-401.

Funding:
• New York City Louis Stokes Alliance for Minority Participation (LSAMP) Program
• CCNY and CUNY start-up funds and CCNY travel funds
• This work was partially supported by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF
Award Number CHE-0117752,and by the New York State Office of Science, Technology, and Academic Research (NYSTAR).