Self-assembly by multi-drop evaporation of carbon-nanotube droplets on a polycarbonate substrate

Abstract

Carbon nanotubes are allowed to self-assemble by depositing a droplet of a water dispersion thereof and letting it evaporate on a polycarbonate substrate. The effect of the number of droplets, evaporated on the same deposition spot, on the self-assembly density is assessed to be more than proportional for the first five depositions. The obtained nanoporous nanostructures are further tested for their electrical resistance and wettability. Two concentrations are used. It is found that a higher concentration and more importantly a higher number of droplet depositions causes the electrical resistance to decrease up to four orders of magnitude and the static contact angle to decrease more than three times. The contact angle hysteresis also increases due to an increasing advancing contact angle and a decreasing receding one. This is explained by the degree of coverage of the substrate by the carbon nanotubes as is also shown by scanning electron microscope images. A better coverage is suggested to cause more pinning for an advancing droplet and a higher capillary force for a receding droplet.Keywords: Multi-drop evaporation, Carbon nanotube droplets, Self-assembled nanostructure, Electrical resistance, Contact angle, Wettability, Contact angle hysteresis IntroductionSelf-assembly has many applications. It is applied in the medical sector [1]. It can also be used for energy storage [2] or in membrane technology [3]. As a principle, the deposition of micro-and nanoparticles on a substrate is relevant for manufacturing different kinds of coatings [4], but also to prepare micro-and nanowires [5] and to deposit complicated organized biological structures, such as DNA [6,7]. There are several ways to achieve particle deposition, such as dip coating, sedimentation and electrostatic assembly. Convective deposition and more particularly drop evaporation are convenient ways to deposit micro-and nanoparticles [8]. The amount of fluid used is minimized, possibly inducing economic advantages, and the outcome can easily be controlled, by choosing initial parameters [9].The interest lies in creating patterned structures out of evaporating drops, which can be of use for energetic and medical applications. The deposited patterns that are left by the evaporated colloidal drops can present a multiplicity of structures, such as the ring structure [10], a central bump [11], a uniform deposit [12], or more complex structures such as multiple rings [13] and hexagonal arrays [5]. This variety of patterns reflects the multiscale attractive forces and transport phenomena taking place during the droplet evaporation. As for the fluid dynamics, several mechanisms play an important role, depending on whether a droplet is deposited directly on the substrate or dropped from a certain height. If droplet impact is associated, the fluid dynamics are determined by the Reynolds and Weber number of the droplet impact [9], as well as the impact angle, interfacial deformation or break-up. Marangoni forces, wetting characteristics and evaporation at ...

Self-assembly of carbon nanotube-based composites by means of evaporation-assisted depositions: Importance of drop-by-drop self-assembly on material properties

“…The nanomaterial will move along the flow patterns and, after evaporation, settle on the substrate, which creates a covered substrate. Drop-by-drop deposition has been studied previously [17], but using only CNTs.…”

“…This can have a large effect on the wettability of the substrate, as well as on the thermal and electrical properties. The effect of self-assembly on the wettability has been studied in a previous work [17]. Here we focus on characterizing the obtained structures for their morphology, electrical and thermal properties.…”

“…In general, these techniques offer great possibilities, but the versatility and controllability of the methods remain to be improved. A rather new technique is to deposit subsequent evaporating drops containing the building blocks of the nanomaterial [17]. The advantage with respect to dip-coating is that an increase of temperature can increase considerably the speed of the procedure due to the controllable size of the droplet and a short injection time.…”

“…The effect of the initial nanoparticle concentration on electrical properties as well as on the presence of the coffee ring have already been studied [17,30]. It appeared that too high concentrations resulted…”