Inkjet Printing of Colloidal Nanospheres: Engineering the Evaporation-Driven Self-Assembly Process to Form Defined Layer Morphologies
© Sowade et al. 2015
Received: 20 July 2015
Accepted: 30 August 2015
Published: 16 September 2015
We report on inkjet printing of aqueous colloidal suspensions containing monodisperse silica and/or polystyrene nanosphere particles and a systematic study of the morphology of the deposits as a function of different parameters during inkjet printing and solvent evaporation. The colloidal suspensions act as a model ink for an understanding of layer formation processes and resulting morphologies in inkjet printing in general. We investigated the influence of the surface energy and the temperature of the substrate, the formulation of the suspensions, and the multi-pass printing aiming for layer stacks on the morphology of the deposits. We explain our findings with models of evaporation-driven self-assembly of the nanosphere particles in a liquid droplet and derive methods to direct the self-assembly processes into distinct one- and two-dimensional deposit morphologies.
Self-assembly processes of molecules or micro- or nanoscopic particles within droplets are an interesting method for the development of ordered assemblies and have attracted considerable interest during the last decades. During the evaporation of the solvents of the droplet, different transport mechanisms force the molecules or particles to certain positions where they assemble and form in part rigid agglomerates. Such explained self-assembly processes are ubiquitous, a natural phenomenon [1–3], and considered also as a promising tool for nanofabrication [4–6]. The resulting deposits, their morphology, and functional properties are very important for many applications, e.g., printing and coating technologies such as inkjet printing, spin coating, or slot-die coating, for paintings and coatings, for bioassay manufacturing, and many others [1, 7–9].
Employing self-assembly processes for molecules or particles within an evaporating patterned or non-patterned liquid film has turned out to be a simple and elegant method to achieve a packing of the dispersed constituents, e.g., for the assembly of ordered structures from colloidal particles in droplets [10–13], one-dimensional lines [14, 15], two-dimensional patterned photonic crystals , or even three-dimensional spherical colloidal assemblies . However, numerous publications show the complexity of the fluid dynamics which the particles and constituents undergo aside of Brownian motion , gravity , and buoyancy during the evaporation. Phenomena such as capillary flows , convectional flows , and nonetheless the attractive (e.g., van der Waals) and repulsive forces (1) amongst particles [8, 16], (2) between particles and substrate [15, 22, 23], and/or (3) at the three-phase boundary (line-tension effects) [19, 24] will contribute to the final shape of the deposit. The deposits obtained by an evaporating droplet containing nanoparticles on non-absorbent and rigid surfaces can range from uniform patterns  to a ring-like pattern via the so-called coffee ring effect , central bumps , and inner coffee ring deposits  or and a number of further patterns in between .
There are two transport mechanisms in literature studied most: (1) capillary flows transporting materials and particles from the center towards the edge where they accumulate due to a pinning of the contact line and (2) inward flows from the droplet edge to the center usually leading to central bumps [2, 8]. For these cases, detailed theoretical and experimental investigations have been made trying to explain the droplet impact, droplet evaporation, and the resulting shape of the deposit [8, 25]. It has been demonstrated that these two transport mechanisms are also of high interest for inkjet printing as well as other direct-writing technologies since they define the morphology of the printed layer and its properties [26–28].
In this research work, following earlier studies on inkjet-printed self-assembled molecular monolayers and self-assembled spherical colloidal assemblies [17, 29], the influence of different parameters on the inward- and outward-directed transport flows of inkjet-printed sessile colloidal droplets on non-absorbent surfaces is investigated focusing on the morphology of deposit. We combine the approach of bottom-up manufacturing based on self-assembly with inkjet printing, a flexible, scalable, and direct-writing deposition technique. Thus, we are presenting a systematic study of the resulting deposits of the inkjet-printed colloidal suspensions as a function of surface energy of the substrate, the temperature of the substrate, and the ink formulation. The monodisperse nanosphere particles of the suspensions serve as a model system for understanding the self-assembly phenomena.
Characteristics of the colloidal suspensions used for the experiments
Duke Scientifics (DS300)
Bangs Laboratories (BL280)
Nanosphere diameter (nm)
305 ± 8
300 ± 5
280 ± 7
Solids content (wt.%)
Surface tension (mN/m)
46.8 ± 0.8
57.3 ± 0.9
70.2 ± 1.5
7.0 ± 0.2
7.0 ± 0.2
7.0 ± 0.2
Measured contact angle of sessile droplets of deionized water on the differently treated glass substrates
Contact angle (°)
67.7 ± 2.7
78.7 ± 1.5
100 ± 5
The colloidal suspensions were printed using a Dimatix DMP 2831 laboratory drop-on-demand (DoD) inkjet printer (Fujifilm Dimatix Inc., Santa Clara, USA). The inkjet printheads have a nozzle diameter of 21.5 μm and a nominal drop volume of 10 pL. The DMP was applied in both single nozzle and multi nozzle modes. The clear distance between the nozzle and the substrate was maintained at 1 mm during printing. All samples were printed at ambient conditions (laboratory conditions 22.5 ± 0.8 °C and 22 ± 3 % relative humidity).
The printed deposits were analyzed by scanning electron microscopy (SEM) using a Hitachi TM-1000 (Hitachi High-Technologies Cooperation, Tokyo, Japan). To avoid the charging effect on the insulating nanospheres, the samples were coated with an about 18-nm-thick layer of Pt by sputtering at 40 mA for 120 s using a BAL-TEC SCD 050 (formerly BAL-TEC AG, Balzers, Liechtenstein) electron microscope preparation system. Optical microscopy analysis was carried out on a Leica DM 4000 M (Leica Microsystems CMS GmbH, Wetzlar, Germany).
Results and Discussion
Influence of Substrate Surface Energy on Droplet Deposit Morphology
As it can be seen from Fig. 2a, it turns out that the higher the water contact angle on the substrates, the lower the diameter of the resulting droplet deposit is. The surface treatment also strongly affects the morphology of the deposit by virtue of the packing of the nanospherical constituents as indicated in Fig. 2b, c as well as in Additional file 1: Figure S1. At low water contact angles (high surface energy of the substrate), resulting capillary flows will transport the nanospheres from the center of the evaporating droplet towards the edge where they agglomerate. This phenomenon is described as coffee ring effect and well observed, investigated, and sometimes also exploited for certain applications many times in literature [5, 8, 20, 27, 28]. At high contact angles (low surface energy of the substrate), no coffee ring effect can be seen but the nanosphere particles are stacked in multilayers. Even colloidal hemispheres (printed domes) can be obtained by printing or dispensing if the substrate has a very low surface energy enabling a high receding contact angle and thus a freely sliding three-phase contact line (see Additional file 1: Figure S2) [10–12, 30]. In our experiments, the area coverage with nanosphere particles of single, isolated inkjet droplets was calculated on the basis of the SEM images by determining the ratio of the number of pixels circumscribed by the deposited particles and the number of pixels making up the total droplet footprint marking the deposit area. Alike in the previous case, it turns out here that the lower the water contact angle and the higher the spreading of the inkjet-printed droplet on the substrate, the lower the particle area coverage is (see Additional file 1: Figure S3). A coverage of 100 % was obtained for BS305 on HMDS- and OTS-treated substrates indicating a continuously covered area with at least one monolayer of nanospheres.
Qualitative summary of main trends based on correlations between printing results and varying printing parameters
Water contact angle
Diameter of the deposits
Circularity of the deposits
Number of layers of the nanospheres
Degree of manifestation of coffee ring effect
Particle area coverage of the circumscribed deposit area
Counteracting the Coffee Ring Effect and Printed Layer Stacks
Aside of the usage of glass substrates with an high contact angle, one can also counteract the coffee ring effect by tuning the ink composition, e.g., by adding high boiling point solvents [11, 31, 32] to decrease the evaporation rate. However, the evaporation behavior becomes very complex for these kinds of co-solvent/surfactant/particle substrate systems since they are considered as non-equilibrium processes . Surfactants are mainly part of the polymer nanosphere suspensions because they are incorporated during the emulsion polymerization process (see surface tension measurement in Table 1, high surface tension indicates low amount of surfactants and vice versa ). It has been reported that the dissolved surfactants may absorb on the substrate and on the nanospheres as well as on the air/solution interface resulting in alteration of the initial contact angle and contact area of the sessile droplet. Surfactants also affect the pinning conditions of the three-phase contact line and all factors finally influence the deposit morphology [9, 33]. The co-solvent which we will add strongly influences the evaporation rate of the droplet which is considered one of the main factors for the ordering of the particles . It was found that electrostatic repulsion and van der Waals forces play a minor role for ordering since the attractive capillary forces upon meniscus formation between solvent and nanospheres during evaporation dominate [9, 34].
Influence of Substrate Temperature on Droplet Deposit
There is not a very big difference comparing the deposit morphology at 35 °C substrate temperature (Fig. 5b) and 60 °C substrate temperature (Fig. 5c). The coffee ring effect takes place at both conditions indicating a pinned contact line. The extension of the contact line is limited in the case of high substrate temperature so that the deposit diameter is finally less. Compared to the deposit at 35 °C, a higher amount of particles inside the coffee ring at 60 °C is remarkable. This observation usually indicates that the coffee ring effect is less pronounced compared to droplet samples without particles in the center. However, we assume that the convective flows resulting in the coffee ring deposit were interrupted due to the high temperature of the substrates. The solvent evaporation of a sessile droplet on the substrate with a nominal volume of 10 pL takes place in less than 400 ms (approximation based on Belgardt et al. ). This short evaporation time prevents further movement of the particles to the edge of the droplet.
Close-Packed Order vs Random Order
Colloidal suspensions were applied as a model ink in inkjet printing to study the layer formation processes based on self-assembly of nanosphere particles on non-absorbent substrates, which are usually employed in the field of printed electronics. It can be concluded that the three parameters under consideration (1) substrate wettability (the surface energy of the substrate represented by the contact angle with water), (2) the ink formulation, and (3) the temperature of the substrate are very important parameters in inkjet printing defining the morphology of the deposits. By tuning these parameters carefully, desired deposit morphologies for certain applications can be obtained, e.g., deposits with a coffee ring shape, highly ordered monolayer deposits with close-packed colloidal arrays, randomly arranged particles in a droplet deposit and multilayer stacks. Our findings highlight the importance of a tuned interaction between ink, constituents, and substrate surface in inkjet printing and any liquid deposition method towards the formation of micro- and nanoscopically engineered morphologies.
The authors thank Torsten Jagemann und Michael Hietschold (TU Chemnitz, Department of Solid Surfaces Analysis) for the support of sample preparation for SEM. Susann Ebert and Werner Goedel (TU Chemnitz, Department of Physical Chemistry) are acknowledged for providing the HMDS and OTS treatment of the substrates. Christoph Sternkiker and Tobias Seifert supported with inkjet printing of the colloidal suspensions. Jolke Perelaer and Ulrich S. Schubert (FSU Jena, Institute for Organic and Macromolecular Chemistry) thankfully provided the silica nanospheres (Bangs Laboratories). The authors acknowledge partial financial support from the EU-FP7 NoE PolyNet (grant agreement 214006).
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