Surface-Tension-Induced Synthesis of Complex Particles Using Confined Polymeric Fluids

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Entry by Pichet Adstamongkonkul, AP 225, Fall 2011

Reference:

Title: Surface-Tension-Induced Synthesis of Complex Particles Using Confined Polymeric Fluids

Authors: Chang-Hyung Choi, Jinkee Lee, Kisun Yoon, Anubhav Tripathi, Howard A. Stone, David A. Weitz, and Chang-Soo Lee

Journal: Angewandte Chemie International Edition, 2010, Vol. 49, No. 42

Keywords: wettability, hydrophobicity, contact angle, interfaces, surface tension, microparticles


Summary

There are attempts to manipulate the physical and chemical properties of polymeric particles to optimize their functions and applications. Conventional approaches include self-assembly, photolithography, stretching/deformation of spherical particles, microfluidics, and nonwetting template molding. However, more complex shapes are also more difficult to handle in a controlled manner. This paper demonstrates a novel method for synthesizing monodisperse particles with a variety of shapes and sizes via "surface-tension-induced flow". The authors loaded two solutions, a photocurable solution (polyethylene glycol diacrylate; PEG-DA) and a nonphotocurable wetting solution (n-hexadecane), into a micromold. By changing the loading sequence of the two solutions, one can form particles with different curvatures and aspect ratio through different contacting interfaces between soltuions, micromold wall surface, and air. Eventually, the photocurable solution is polymerized using UV light.

Advantages and Limitations of the Conventional Methods

Most current methods are limited to two-dimensional or spherical shapes, since the more complex shapes are harder to handle.

  • Bottom-up approaches
    • Based on self-assembly mechanisms such as
      • Liposome preparation
      • Heterogeneous polymerization
      • Colloid synthesis
    • Difficult to manipulate to control morphology and structure
  • Top-down approaches
    • Such as Photolithography
    • Inherently limited by the availability of materials - photolithography is not compatible with organic materials as the technique involves using harsh solvent in wet etching, high energy in ion etching, high-temperature baking, multiple steps for layers removal, and strong energy deposition.
  • Microfluidic platforms
    • Allows the formation of spherical, disks, plugs, rods according to the microchannels or photomask geometries and flow conditions
    • Several limitations
      • Fast solidification without deformation and channel adhesion are required
      • Morphologies of particles are limited by the channel or photomask geometries
  • Particle Replication In Nonwetting Templates (PRINT)
    • Developed to fabricate monodisperse particles with a wide range of size and shape
    • Reproducible and easy processing
    • A variety of materials can be used
    • Still difficult to produce three-dimensional shapes

Surface-Tension-Induced Flow Methodology

Surface-tension-induced-1.jpg

This technique enable us to control the curvature of the top and the aspect ratio of the forming particles from two reasons. First, the capillary effect of the wetting solution and the formation of interfaces depend on the loading sequence. Secondly, the difference in densities of the solutions also determine the shape. Finally, the aspect ratio of the mold itself governs the resulting shape and size. In sequence A depicted in the figure, PEG-DA was added on a patterned PDMS micromold and the excess was removed, and the solution forms a contact angle at the interfaces. Then the n-hexadecane solution was added on top. The hydrophobicity of the wetting solution promotes higher wettability on the PDMS wall than the PEG-DA, which induces the capillary force on the mold surface. Consequently, the n-hexadecane moves down the well, pushing the PEG-DA solution away from the wall. Since the PEG-DA solution has to maintain its volume, the decrease in the width increases the height of the solution column. The top part of the PEG-DA forms the interface with the wetting solution and, to minimize the energy, convexes upward.

In contrast, in sequence B, when n-hexadecane was loaded first, the solution preferentially wets the PDMS surface. When PEG-DA solution was added, its higher density pushes the wetting solution from the PDMS mold, leaving behind a thin layer of wetting solution sticking on the well surface. This results in a flat top particle. These particles can be released from by bending the mold. These two sequences confirm the controllability of the curvature of the particles and enable the high-throughput formation of anisotropic particles.

Actual-AB.jpg

As a proof of concept, the authors also fabricated several other shapes, depending on different sequence and the shape of the mold used. Some shapes include hearts, hexagons, twin cylinders, and fused twin donuts.

Shapes.jpg

The authors mentioned the advantage of this method over PRINT in creating the curved-top anisotropic particles from sequence A, in which the latter method could not achieve such curved shape from soft lithography. The fabrication of particles with one closed, curved-top end is also another forte of this approach. They also demonstrated the fabrication of Janus particles, whose hemispheres have different compositions or shapes. In this study, three types of JAnus particles can be fabricated using this method:

  • Concave-flat Janus particles made by sequentially fabricate the particles following sequence A, then B.
  • Flat-flat Janus particles made with sequential combination of two sequence-B processes in a plane parallel to the well bottom.
  • Flat-concave Janus particles made from the combination of sequences B, then A.

By incorporation of fluorescent dyes into each solution, one can visualize the different compartments of the made particles, which could be further used in encapsulation of agents or selectively chemical functionalization of the compartments,

Janus.jpg

As a result, we can control and vary the three-dimensional shapes of the particles using the same mold, as well as manipulate the curvature of the particles. This method also allows high throughput screening and production rate. Finally, multiple components can be formed without the needs of additional steps or devices.