Convective Assembly

This equation establishes a relation between the growth velocity of the layer Vc, the porosity e, the height of the assembled layer h, the volume fraction ratio of the particles in the suspension tp, a coefficient that relates the solvent velocity to the particle velocity /J(0 p 1), and the evaporation rate of pure water /e-The variable l[x] is defined as I = Jev3.g/Je[x), where/evap is the integral of water evaporation over the x-axis and e(x) is the evaporation rate at any point of the drying region (Fig. 15.6a). The x-axis is parallel to the substrate and perpendicular to the contact line. 

Equation 15.1 shows that three key process parameters can be used to control the coating thickness and thus its structure (i) the particle volume fraction 0, (ii) the solvent evaporation rate Jevap, and (iii) the substrate velocity Vs This latter parameter does not appear explicitly in Eq. 15.1 which refers only to a steady state situation. Vs is an experimental parameter. Its value should be set close to the Vc value to ensure a stable assembly process. For a given evaporation rate and volume fraction, a uniform crystalline monolayer with a defined thickness is obtained if Vg = Vc so that the particle influx toward the meniscus exactly compensates 

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Wargacki SP, Pate B, Vaia RA (2008) Fabrication of 2D ordered films of tobacco mosaic virus (TMV) processing morphology correlations for convective assembly. Langmuir 24 5439-5444... 

Wolf et al. introduced the self-assembly, transfer, and integration (S ATI) of nanoparticles with high placement accuracy.86 87 Silica and polymer nanoparticles were positioned on a PDMS stamp through convective assembly (Fig. 13.14). By controlling the printing temperature or by using a thin polymer layer as an adhesion layer, nanoparticles of different shapes and sizes were printed onto the target substrate. 

Figure 13.14 The use of convective assembly to control the arrangement of nanoparticles on apattemed PDMS stamp and the subsequent printing of the nanopaiticles onto a substrate with single-particle resolution.87...

Dimitrov, A.S. and Nagayama, K., Continuous convective assembling of fine particles into two-dimensional arrays on sohd surfaces, Langmuir, 12, 1303, 1996. 

G. Prevo, D. M. Kuncicky, and O. D. Velev, Engineered Deposition of Coatings from Nano- and Micro-Particles A Brief Review of Convective Assembly at High Volume Fraction, Colloidal and surface A, 311, 2-10 (2007). 

FIGURE 54.7 Mechanism of convective assembly on (a) flat and (b) vertical surfaces. (Adapted from Nagayama, K Coll. Surf. A, 109, 363, 1996.)... 

At the nanoscale, colloidal assembly systems rely on fluid flows caused by natural COTivection for positioning colloidal particles. Convective assembly patterns are controlled by evaporation rate and contact line geometry. Coatings with monolayer precision have been created through... 

Sdf-Assembly. Gravitational ass bly was performed on silicon tinplates with 60 pm de holes defined through standard photolithography that were coated with a perfluorodecyltTichlorosilane monolayer. Particles were d x>sited on the coated tonplat which was then placed in a sealed ceU. Ihe entire setup was excited at 60-100 Hz so that the amplitude was just sufficient to fluidize the particles. When all holes in the tinplate were filled with particles, the template was removed, and excess particles were picked up by a carrier that did not touch the assembled particles. Convective assembly (15] was used to order 500 nm polystyrene particles into 2D continuous layers. It was performed on flat PDMS templates previously oxidized by oxygen plasma. Capillary assembly [14] provided 500 nm polystyrene particle ass nblies with full control of the positions on patterned PDMS templates without any surface modification. PDMS templates were fabricated by injection molding on flat or patterned silicon masters [32]. They consisted of a 200 pm thick layer of custom-... 

Using Convective Flows for the Growth of 2D Layers Convective Assembly... 

An example of an experimental setup for such an experiment will be described in section 15.2.3. On a flat surface, convective assembly leads spontaneously to extended ordered layers or multilayers (Fig. 15.2). In many applications, the control of the position and lattice parameters would be of great interest. Intuitively, combining... 

Figure 15.2 Examples of 2D layers fabricated by convective assembly, [a] Examples of a crystal monolayer obtained from 1.1 jam PS latex particles. The crystalline order leadingto bright diffraction is revealed when illuminated with a white light from behind. [b) SEM images of a 100 nm gold nanoparticles monolayer assembled on a silicon surface (unpublished results], a monolayer (c], and a bilayer (d] obtained with 500 nm PS particles. The structures were first assembled on a flat PDMS substrate and then transferred onto a silicon wafer by printing. Abbreviations SEM, scanning electron microscopy PDMS, polydimethylsiloxane PS, polystyrene.

The choice of the experimental setup is determinant and has to be done according to the complexity of the structure to be achieved. Many simple methods can provide straightforward routes for the manufacturing of extended 2D layers through convective assembly. However, the control of the precise positioning of objects in the capillary assembly mode is more restrictive and usually requires a more accurate control of the evaporation rate and/or control of the meniscus velocity. Systems that are capable of decoupling these two parameters are more flexible and offer versatile platforms assembly methods. The objective of this paragraph is to present an overview of the systems reported in literature and to help the reader to select the approach the more appropriate to its application with a balanced investment/performance ratio. 

First demonstrations of the convective assembly mechanism were performed on static evaporating droplets.In most experiments using this simple but still efficient approach (Fig. 15.4), the control of the shape of the meniscus is usually used to define the number of layers of the assembled 2D array. - - " ... 

Figure 15.6 (a] Typical optical image acquired during the assembly of 500 nm PS particles (0.2% solid content, Ts 18°C) on a flat PDMS substrate treated with oxygen plasma. The left-hand side of the image shows the assembled layer, whereas the right-hand side shows the liquid suspension of particles. A schematic illustration of the evaporation process mechanism in convective assembly is given. (b) Temperature and solid content dependence of the convective assembly speed (obtained for 500 nm PS partlcles]. ... 

Figure 15.7 Influence of the evaporation rate on the convective flow involved in convective assembly. Experiment was performed on an oxygen plasma-treated PDMS surface [500 nm PS particles, 0.2% solid content].

The existence of three different regimes, namely, assembly, stagnation, and disassembly, and the reversibility of the whole mechanism are clearly a convenient feature in convective assembly. 

Convective assembly can be combined with templated substrates either to define areas of assembly or to modify the structural properties of the assembled layers. 

The use of patterned templates with small-dimension structures is usually not associated to convective assembly indeed, the introduction of topology may disturb the assembly process as each pattern can potentially behave as an obstacle that impedes particles motion toward the contact line. Nevertheless some experiments have demonstrated that the use of shallow patterns preserves the convective assembly mechanism as long as i) the liquid film above the structures is high enough and ii) the pattern density is sufficiently low to allow particles to move around the patterns. Figure 15.9 shows an example of structures assembled on a PDMS layer. 

Figure 15.9 Illustration of the templated convective assembly mechanism 500 nm PS particles were assembled on a PDMS substrate [Ts = 20°C Vs = 1.5 p.m.s with 200 nm deep patterns] [unpublished results]. The assembled structures were transferred on a silicon wafer by printing. ...

To be effective, convective assembly requires free suspended particles that can be easily dragged by hydrodynamic forces. In the previous paragraphs, we have already shown that the convective assembly mechanism is perfectly compatible with PS particles in the micrometer range. 

Whereas the behavior of the meniscus is different in both assembly mechanisms, evaporation plays a critical role in capillary assembly as well. Hydrodynamic flows created by evaporation tend to bring particles from the bulk suspension toward the contact line. In convective assembly, this accumulation of particles close to the contact line results directly in layer formation. In capillary assembly, even if capillary forces prevent deposition, it was recently shown that this process was essential to initiate particle deposition. - ... 

The creation of this accumulation region seems to be strongly dependent on the evaporation of the solvent and thus can be directly controlled by an appropriate tuning of the temperature close to the contact line. Indeed, as it was observed for convective assembly, evaporation induces a particle flow Ji from the suspension to the drying region. Accumulation of particles occurs when particle consumption through deposition does not compensate for particle income. This mechanism is illustrated in Fig 15.11 in the case of 500 nm PS particles. 

Once again, it is possible to establish a temperature threshold value for the assembly mechanism. However, whereas this threshold value was matching dew point temperature in the case of convective assembly, experimental observations of capillary assembly show that this threshold value is well above the dew point value. This means that considering both the influence of particle influx due to evaporation and particle deposition is not sufficient to illustrate mass transfer balance. 

This result was demonstrated by monitoring the influence of adding a sodium dodecyl sulfate (SDS)/TritonX45 surfactant mixture, while keeping all other experimental parameters constant. Increasing the surfactant concentration resulted in a decrease of the contact angle. For contact angle values below 15°-20°, the process switches from capillary to convective assembly below 20°. Indeed,... 

The question of pattern shape and its influence on the wetting properties and trapping efficiency is more complex. In the case of a high-aspect-ratio structure, wetting properties are affected by capillary pressure, which can lead, in some extreme cases, to the conversion of capillary assembly into convective assembly. This case will be discussed in section 15.4.4. 

This criterion is critical for convective assembly. It seems that it is less restrictive for capillary assembly. Recent studies have demonstrated that even in the case of sedimenting particles, it is still possible to perform capillary assembly, as long as particles are not interacting strongly with the template surface and can be dragged by the mean of capillary forces. Figure 15.16 shows experimental results obtained with 4.5 p.m magnetic particles (Dynal). 

The Ozin group pioneered the fabrication of planarized microphotonic structures and the growth of ordered crystals using evaporation techniques derived from convective assembly. Figure 15.17 illustrates the methods used in these experiments and some examples of PBG structures. " Opened microchannel templates were immersed vertically in a suspension of silica microspheres. The assembly was then driven by the evaporation of the colloidal suspension. Whereas the assembly of 100-500 nm... 

It seems that two parameters are essential to benefit from these assembly mechanisms. Obviously, the control of the substrate velocity is needed to control the meniscus motion and to perform the assembly over large areas. In the two processes investigated, it has been shown also that controlling the hydrod3mamic flows created by evaporation is critical and can easily be achieved by adjusting the substrate temperature. In convective assembly, the temperature allows fast and accurate control of the deposition speed, including reversal of the assembly process, whereas in capillary assembly, a distinct temperature threshold has been found that triggered the assembly. 

B. G. Prevo, J. C. Fuller, and 0. D. Velev, Rapid deposition of gold nanopaiticle films with controlled thickness and structure by convective assembly, Chem. Mater., 17, 28-35 [2005],... 

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