Particle assembly

We aim to control the positioning and deposition of colloidal objects that range from several 10 nanometers to a few micrometers in size. Capillary forces at the edge of a moving meniscus are very suitable to position large numbers of colloidal objects in parallel into predefined trapping sites on a template.

In a sequential assembly process, new colloidal materials — known as “colloidal molecules” — can be produced with an unprecedented freedom of composition and shape. Such colloidal molecules are very promising model systems for molecular assembly processes and they are building blocks for actively and autonomously moving colloidal objects that can mimic active living matter.


Science Advances, 2(4), e1501779 (2016).

Programmable colloidal molecules from sequential capillarity-assisted particle assembly
S. Ni, J. Leemann, I. Buttinoni, L. Isa, H. Wolf
Science Advances, 2(4), e1501779 (2016).
The assembly of artificial nanostructured and microstructured materials which display structures and functionalities that mimic nature’s complexity requires building blocks with specific and directional interactions, analogous to those displayed at the molecular level. Despite remarkable progress in synthesizing “patchy” particles encoding anisotropic interactions, most current methods are restricted to integrating up to two compositional patches on a single “molecule” and to objects with simple shapes. Currently, decoupling functionality and shape to achieve full compositional and geometrical programmability remains an elusive task. We use sequential capillarity-assisted particle assembly which uniquely fulfills the demands described above. This is a new method based on simple, yet essential, adaptations to the well-known capillary assembly of particles over topographical templates. Tuning the depth of the assembly sites (traps) and the surface tension of moving droplets of colloidal suspensions enables controlled stepwise filling of traps to “synthesize” colloidal molecules. After deposition and mechanical linkage, the colloidal molecules can be dispersed in a solvent. We demonstrate the “synthesis” of a library of structures, ranging from dumbbells and triangles to units resembling bar codes, block copolymers, surfactants, and three-dimensional chiral objects.

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Heiko Wolf

Heiko Wolf

IBM Research scientist


Joint study with Prof. Dr. Lucio Isa, ETH Zurich.

Science Advances, 2(4), e1501779 (2016).

Insights into mechanisms of capillary assembly
S. Ni, J. Leemann, H. Wolf, L. Isa
Faraday Discuss., 181, 225–242 ( 2015).
Capillary assembly in a topographical template is a powerful and flexible method for fabricating complex and programmable particle assemblies. To date, very little attention has been paid to the effects that the trap geometry – in particular the trap depth – has on the outcome of the assembly process. In this paper, we provide insights into the mechanisms behind this directed assembly method by systematically studying the impact of the trap depth and the surface tension of the suspension. Using confocal microscopy, we investigate the assembly process at the single-particle level and use these observations to formulate a simple mechanical model that offers guidelines for the successful assembly of single or multiple particles in a trap. In particular, single particles are assembled for shallow traps and moderate surface tensions, opening up the possibility to fabricate multifunctional particle dimers in two consecutive assembly steps.

Microelectronic Engineering, 141, 1216 (2015).

Capillary assembly of cross-gradient particle arrays using a microfluidic chip
S. Ni, M.J.K. Klein, N.D. Spencer, H. Wolf
Microelectronic Engineering, 141, 12–16 (2015).
Arrays with well-defined particle registration are of high importance in device fabrication for biosensors, electronics and optics. Also, materials exhibiting gradient variations of properties (e.g. wettability) in one, two or three dimensions have proven their capability for high-throughput screening of various interactions (e.g. cell–surface interactions). Here, we present the fabrication of cross-gradient particle arrays (CGPA) featuring a gradual cross-over from one particle type to another, while keeping the overall particle density constant. CGPAs were prepared by means of a capillary assembly setup assisted by a micro-fluidic chip. This setup offers a high level of control over the capillary assembly process with respect to the composition and location of the assembled arrays. The resulting two complementary gradients may be used in combinatorial studies of biological and chemical interactions. We demonstrate the application of a CGPA as a two-level security feature with unclonable finger-print-like patterns. Moreover, we show the possibility of obtaining a 2D CGPA by capitalizing on diffusive transport within the capillary bridge perpendicular to the assembly direction.

Langmuir, 30(1), 9095 (2014).

Cascaded Assembly of Complex Multiparticle Patterns
S. Ni, M.J.K. Klein, N.D. Spencer, H. Wolf
Langmuir, 30(1), 90–95 (2014).
A method for the cascaded capillary assembly of different particle populations in a single assembly cycle is presented. The method addresses the increasing need for fast and simple fabrication of multicomponent arrays from colloidal micro- and nanoscale building blocks for constructing nano-electronic, optical, and sensing devices. It is based on the use of a microfluidic device from which two independent capillary bridges extend. The menisci of the capillary bridges are pulled over a template with trapping sites that receive the colloidal particles. We describe the parameters for simultaneous, high-yield assembly from both menisci and demonstrate the applicability of the process by means of the size-selective assembly of particles of different diameters and also by the fabrication of two-component particle clusters with defined shape and composition. This approach allows the fabrication of multifunctional particle clusters having different functionalities at predetermined positions.

Rev. Sci. Instrum., 83(8), 86109 (2012).

A microfluidic chip setup for capillarity-assisted particle assembly
M.J.K. Klein, C. Kuemin, T. Tamulevicius, M. Manning, H. Wolf
Rev. Sci. Instrum., 83(8), 86109 (2012).
We developed a microfluidic chip setup for capillarity-assisted particle assembly (CAPA). A capillary bridge is formed between the aperture of a silicon chip and the assembly template. The bridge is fed with particle suspension through a microfluidic channel on the chip top side. With this setup, we can control the particle assembly location and tune the suspension composition during particle assembly. In this note, we describe the chip setup, the CAPA process using the microfluidic chip, and results of complex particle assemblies, such as composite particle arrays and particle gradients, that could not be obtained using a conventional CAPA setup.

Advanced Functional Materials, 22(4), 702708 (2012).

Oriented Assembly of Gold Nanorods on the Single-Particle Level
C. Kuemin, L. Nowack, L. Bozano, N.D. Spencer, H. Wolf
Advanced Functional Materials, 22(4), 702–708 (2012).
Non-spherical colloidal nanoparticles have great potential for applications owing to their enhanced directional properties. However, the lack of methods to precisely assemble them on surfaces has hindered exploitation of their properties for planar devices. Here, the oriented assembly of short gold nanorods with lengths below 100 nm from colloidal suspensions is demonstrated. A locally induced phase transition confines the colloidal nanorods at a receding three-phase contact line that is controllably moved over a nano structured surface in a capillary assembly process. Dedicated topographical trapping sites allow for aligned assembly of the nanorods on the single-particle level. The feasibility of this method is demonstrated by assembling nanorods into long-range-ordered, non-close packed arrays that could serve as anti-counterfeit labels by virtue of their distinct optical appearance in the far-field. Furthermore, oriented nanorod dimers that are deterministically assembled have the potential to function as nano-plasmonic antenna devices.