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Magnetic Field-Driven Convection for Directed Surface Patterning of Colloids

Journal Article · · Langmuir
 [1];  [2];  [3];  [3]
  1. Louisiana State University, Baton Rouge, LA (United States); Louisiana State University
  2. Louisiana State University, Baton Rouge, LA (United States); University of Louisiana, Lafayette, LA (United States)
  3. Louisiana State University, Baton Rouge, LA (United States)
Drying sessile droplets is a promising route to transform colloidal dispersions into surface coatings, which are widely used in material design and biochemical detection. However, directing the assembly of the particles within drying droplets and achieving surface patterns beyond the well-known coffee-ring formation remain a challenge. Here, we present a new principle of directing the assembly of nonmagnetic colloidal particles dispersed in a magnetic fluid and generating unusual surface patterns. We use the ability of ferrofluids to change phases with the application of magnetic fields to program the assembly of nonmagnetic microparticles present in drying sessile droplets. We show that in the absence of external magnetic field, the superparamagnetic nanoparticles in the magnetic fluid are spontaneously transported to the droplet edge because of solvent evaporation. This nanoparticle transport leads to the formation of nanoparticle-rich edge and nanoparticle-depleted center of the drying droplet. Upon the application of a uniform external magnetic field, the asymmetry in the magnetic nanoparticle distribution drives a magnetostatic convection and finger-like instability from the droplet edge to the center. This magnetic microconvection from droplet edge-to-center reverses the particle transport from center-to-edge, well-known for drying droplets in the absence of external field. We use this magnetostatic microconvection to assemble secondary nonmagnetic microspheres in droplets, overwriting ring formation and direct their assembly into four distinct kinetically stable states. As a result, the method presented here offers an active control over the colloidal assembly achieved by drying sessile droplets and thus enables a new route for fabricating complex patterns and functional surface coating.
Research Organization:
Louisiana State Univ., Baton Rouge, LA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
SC0012432
OSTI ID:
1670774
Alternate ID(s):
OSTI ID: 1611792
OSTI ID: 1674948
Journal Information:
Langmuir, Journal Name: Langmuir Journal Issue: 50 Vol. 34; ISSN 0743-7463
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English

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Cited By (1)

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Figures / Tables (5)