~ PhD Research Projects


General interests :
Atomic Force Microscopy (AFM) in liquid environments.
Atomic Force Microscopy (AFM) studies of friction and lubrication.
Electronic properties of interfaces at the Nanoscale with AFM and STM.
Flow of liquid in a single nanopore.

Contact:
A/P Thomas Liew elelyf@nus.edu.sg
Dr. Sean O’shea s-oshea@imre.a-star.edu.sg 

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Project 20160916A : Mesoscale friction and contact at very low load

The mechanical contact between two surfaces is critical for many applications, such as the integrity of electrical connection, the sliding and adhesion of surfaces, the formation of charge barriers for electronic devices, and the influence of surface roughness. This project will relate friction and the associated contact mechanics at the nanometre scale to friction on mesoscale structures at very low loads. Such data is important for verifying theoretical models of how friction evolves from a single Nanoscale asperity (the basic starting point of theoretical studies) to real macroscopic structures which contain many asperities in contact. Moreover, Nanoscale contact mechanics is critical for microfabricated devices and the new topic of the mechanical behaviour of 2D materials.

Our key ideas here are based around a fundamental question: What happens within a macroscopic sized region just at the very initial contact of two surfaces? That is, at very low load.

There are many theoretical developments on this problem (ours included!) but only two experimental studies to date. We will address this short coming by directly measuring the initial contact and friction between mesoscopic plates (Si and 2D plates of dimensions 0.1 to 20 microns) placed on rough Si surfaces (see Figure below). Scanning probe microscopy methods (AFM and STM) and Transmission electron microscopy (TEM) will be the major experimental methods used. Experimentally, the top surface must be as thin as possible (e.g. 2D) to increase the spatial resolution of TEM, AFM or STM. With this approach we can begin to answer some critical and fundamental questions of the contact of two surface, such as : Can we directly measure the number of asperities in mechanical contact between surfaces? How many asperities are there and how do they deform? Which theory best describes the condition of initial contact? How do the asperities and friction change with increased load and does this dependence follow expected theory?








Project 20160916B : Nanoscale markers to measure Nanoscale wear

Wear of material from one surface during sliding is the most detrimental aspect in real technological applications, yet remains one of the most difficult mechanisms to quantify at a fundamental level. One reason for this is it is extremely difficult to directly measure the wearing of a single Nanoscale asperity as it is occurring.

We will address this problem by using Atomic Force Microscopy (AFM) in which the AFM tip acts as the single asperity in contact with another surface (see Figure below). The AFM tip itself is either formed from, or coated with, a material which emits light e.g. a polymer coating with fluorophores or a diamond tip with nitrogen vacancy spin centres. The AFM is mounted on an optical stage to detect light emission and the premise is the time evolution of the emitted light intensity or wavelength can be used to infer the wear rate of material at the tip. For example, in diamond the emission is extremely sensitive to the location of the nitrogen vacancy with respect to the surface. The light emitters act as Nanoscale markers and by this means we can monitor wear as a function of time under different load conditions, and compare with fundamental wear rate theories.

           

 

Project 20160916C : Microfabricated fluidics for nanopore flow measurement

The flow of electrolyte e.g. ions in water, through pores as small as 1 nanometre can be measured easily and with accuracy by measuring the ion current flow. This simple yet powerful method has provided immense insight into how water flows through nanopore channels, with obvious significance to problems in desalination and water purification.

However, there is also a need for methods that are not based on this standard means of measuring the ion-current because many liquids simply have too low conductivity e.g. oils, alcohols, even deionised water. The flow of non- aqueous liquids in nanopores is important in technological areas such as oil extraction and filtration processing.

In this project we will design and construct MEMS based devices to measure fluid flow in a single nano-scale pore. An example is to scale down volume type macroscopic measurements to the micron scale (see Figure below). Liquid flow through a single nanopore in a membrane changes the liquid volume on one side of the membrane, which is measured with a micron sized version of a manometer. The flow can also be cross checked against the standard ion-current method using water. Specific or alternative designs must be developed, but rough calculations show that suitably constructed fluid channels or chambers of dimensions ~10’s microns should suffice to observe changes in fluid volume arising from single nanopore flow.