Nano-objects are continually studied in tribological applications and increasingly in other applications that require controlled manipulation and targeting in liquid environments. The need for suitable forms of lubrication for micro/nanoelectromechanical systems (MEMS/NEMS) and the ability to control and transport nano-objects in liquids, requires an understanding of nano-object behavior, with regards to friction, adhesion and wear, which is essential to their successful and continued application.
Increasing the lifetime and efficiency of individual components of systems is crucial to the commercialization of MEMS/NEMS [1
]. As one moves from the macroscale to the micro/nanoscale, surface to volume ratio increases. Therefore, adhesive and friction forces, which are dependent on surface area, become more significant. With MEMS/NEMS devices, the initial start-up forces and torques needed become high, which can hinder device operation and reliability [2
]. The choice of a suitable lubricant on these scales becomes crucial.
Nano-objects are also used for applications that require controlled manipulation and targeting mechanisms in biomedicine and the oil industry. Applications include, but are not limited to, their use in targeted drug delivery and chemical sensors in the identification of oil, removal of contaminants and enhanced oil recovery (EOR). Au, iron oxide, polymer and silica nanoparticles have been studied in targeted drug delivery [3
]. In cancer treatment, nanoparticles are either functionalized with biomolecules that recognize and attach to the cancer cells, [6
] or in the case of iron-oxide nanoparticles, the nanoparticles are directed by an external magnetic field [9
]. The cells are destroyed by drugs that coat the nanoparticles or by increasing the temperature of the nanoparticles to which cancer cells are susceptible. shows a nanoparticle loaded with a therapeutic drug and functionalized with a biomolecule (ligand), which selectively attaches to receptors in the cancer cell. The drug is then released as the nanoparticle diffuses into the diseased cell resulting in cell death.
Figure 1 (a) Schematic of drug-carrying nanoparticles targeting cancer cells and releasing their therapeutic payload resulting in death of the cancer cell. Reprinted by permission from Macmillan Publishers Ltd , copyright 2011. (b) Showing the process of oil (more ...)
Several factors need to be considered for the successful use of nanoparticles in targeted drug delivery. Biological barriers, including physical surfaces and the reticulo-endothelial system (RES), which detects and sequesters blood-borne particles, can prevent nanoparticles from reaching their intended target [7
]. Smaller nanoparticles can diffuse through surfaces and avoid detection by the RES. Studies have shown that forces such as hydrodynamic and van der Waals forces along with the nanoparticle size influence lateral drift (margination) and adhesion to cell walls [5
], which are important factors for effective drug delivery.
In oil-detection studies, as in the example shown in , oxidized carbon-black nanoparticles with a polyvinyl alcohol shell are coated with an oil-detecting agent (2,2’,5,5’-tetrachlorobiphenyl (PCB)). The release of this agent on contact with hydrocarbons is used as an indication of the presence of oil on recovery of the nanoparticles [10
]. In contaminant removal, nanocomposites composed of collagen and superparamagnetic iron-oxide nanoparticles (SPIONs) have been investigated. The collagen selectively absorbs the oil by motion of the nanoparticles towards the oil in a magnetic field [12
]. Magnetic nanoparticles are also of interest in enhanced oil recovery (EOR) since they can be dispersed in fluid and manipulated and monitored by an external magnetic field [13
]. In both oil detection and EOR, agglomeration of nanoparticles can prevent flow through porous media. Nanoparticles can adhere to the surface over which they flow, which results in losses and prevents their eventual recovery [12
]. Studies have shown that surface charge can cause nanoparticles in liquids to adhere to sites in porous media and hinder mobility [15
]. Functionalizing nanoparticles with a hydrophilic polymer has been shown to reduce aggregation and improve flow [12
For many of these applications, control of the friction of nanoparticles moving in the fluids, as well as the friction and adhesion as nano-objects come into contact with each other and surfaces present in their working environment, is necessary.
Nano-object additives have proven to be successful in macroscale studies in reducing friction and wear when added to solid materials and base-liquid lubricants and are expected to provide similar benefits on the micro/nanoscale. Some examples of nano-objects in liquids and their reported sizes, for friction and wear reduction, with studies carried out on the macroscale, are as follows: WS2
platelets (0.5 µm) in commercial mineral oil [16
], ferric oxide nanoparticles (20–50 nm) in 500 solvent neutral (SN) mineral oil [17
], spherical MoS2
(15–60 nm) in poly-alpha-olefin (PAO) and 150 SN [18
], spherical WS2
nanoparticles (50–350 nm) in SN 150 and SN 190 [19
], spheroidal carbon-nano-onion nanoparticles (<10 nm) in PAO [20
nanoparticles (120 nm) in paraffin oil [21
spheres (0.5–3 µm) in 500 SN oil [22
] and carbon spheres (420 nm) in water [23
]. Mechanisms for friction and wear reduction have been reported as tribofilm formation, rolling, sliding, and reduced contact area. It is expected that the reduced contact area and mobility offered by nano-objects observed on the macroscale will also lead to friction reduction and wear protection on the micro/nanoscale. These micro/nanoscale contacts are relevant for MEMS/NEMS devices.
In MEMS/NEMS devices, commercial lubricant oils are unacceptable as base liquids on machine components running in liquid. This is due to energy losses associated with the large viscous drag. In experiments where electrostatic micromotors are operated in a liquid environment, there have been problems of excessive drag and damping, which limited operating speeds, due to the use of high viscosity (20–60 cSt) oils [24
]. However, studies have also demonstrated that friction and wear can be reduced with liquids of low viscosities [25
]. Liquids such as glycerol and dodecane have been shown to reduce friction and wear. Glycerol has a dynamic viscosity (934 mPa·s) that is significantly higher than water (0.89 mPa·s) and studies were performed on the macroscale by using pin-on-disk testers [26
]. In these studies, glycerol was also combined with water to lower the viscosity, which may be feasible for micro/nanoscale applications. Dodecane has been used as a base fluid with ZnS nanorod additives [27
], which also resulted in a reduction in the coefficient of friction and wear. Tests were performed by using a surface force apparatus (SFA) with crossed-mica geometry with a 0–1600 µm2
To characterize friction forces associated with controlled manipulation and to understand the nature of the mechanism of friction and wear reduction of nanoparticles in MEMS/NEMS devices, studies have been carried out in both single-nanoparticle contact and multiple-nanoparticle contact with the aid of an AFM. Both mechanisms are described in detail in the following section.
In single-nanoparticle contact, a sharp AFM tip, as shown in as an example, is used to push the nanoparticle laterally (lateral manipulation). Manipulation studies of nanoparticles, with the aid of an AFM have shown that there is a contact-area dependence of the friction force. Several types of nanoparticles with reported diameters, such as latex spheres (80–100 nm) [28
], Sb nanoparticles (120–400 nm) [29
], (50–500 nm) [30
], spherical SiO2
nanoparticles (30 nm) [31
] and spherical Au nanoparticles (25 nm) [32
], (30–50 nm) [31
] and (80 nm) [33
] have been studied in both contact and intermittent-contact modes in dry environments. In liquid environments, Au nanoparticles (20–30 nm) have also been manipulated in water and ethanol with an AFM operated in intermittent-contact mode [34
]. In addition to the contact-area dependence of friction observed in these studies, the relative-humidity (RH) dependence of friction was investigated by Mougin et al. [32
] and Palacio and Bhushan [31
]. In the study by Mougin and co-workers [32
], it was found that Au nanoparticles could not be moved in an ultrahigh vacuum (UHV) as compared to an ambient environment under otherwise identical manipulating conditions. Palacio and Bhushan [31
] found that for larger nanoparticles, the friction force was lower at lower RH (10%) compared to higher RH (40%) for both Au and SiO2
particles. Both studies were performed on silicon substrates. This would suggest that some adsorbed moisture between the nanoparticle and substrate is necessary for enhanced lubricity.
Schematics of (a) a sharp tip pushing a particle in single-particle contact and (b) a glass sphere sliding over several particles in multiple-particle contact.
Manipulation studies of nanoparticles submerged in liquid environments, to simulate nanoscale contacts and characterize friction forces, are limited. Such studies are necessary for simulating the kinds of environments that involve controlled-manipulation and targeting-mechanism applications of nanoparticles. In addition, these studies provide insights into the interactions of single nanoparticles with a surface, in dry and submerged-in-liquid environments.
In addition to determining the friction force due to lateral manipulation, the effect of the normal load on the friction force has also been investigated. In multiple-nanoparticle contact, a glass sphere attached to an AFM cantilever, as shown in as an example, was used to slide over several nanoparticles. This type of study simulates the contacts experienced by MEMS/NEMS devices when nanoparticles are introduced for the purpose of friction and wear reduction.
Previous studies have been performed using a colloidal glass sphere attached to an AFM cantilever on bare silicon surfaces [35
] and in multiple-nanoparticle contact with both immobile asperities on polymer surfaces [36
] and mobile nanoparticles, such as spherical Au and SiO2
nanoparticles on silicon surfaces [31
]. In these studies, friction forces were reduced due to the reduced contact area provided and, in the case of Au and SiO2
, the possible sliding and possible rolling of individual nanoparticles. Similar to single-nanoparticle contact studies, AFM studies of multiple-nanoparticle contacts submerged in a liquid environment are also lacking. These studies are crucial to determine the added advantage of dispersing nanoparticles in liquids, in cases where the entire MEMS/NEMS system is submerged in a liquid environment. This has the ability to eliminate the adhesive effects of meniscus forces associated with the formation of capillary bridges due to adsorbed moisture on a surface.
Objective of this research
In this study, spherical Au nanoparticles are investigated to determine their effect on friction and wear under dry conditions and submerged in water. Lateral manipulation of single nanoparticles with a sharp tip is used to determine the friction force between the nanoparticle and the silicon substrate by AFM. The coefficient of friction is also investigated, with the aid of a glass sphere attached to an AFM cantilever sliding over multiple nanoparticles. Wear tests were performed on the nanoscale by using AFM and on the macroscale by using a ball-on-flat tribometer. This helps to link the nanoscale friction and wear to that observed on the macroscale and to fully understand the mechanisms involve.