Probing nanoscale forces of nature in fluid

Abstract

Nanoscale forces from natural phenomena are hard to measure. It is not insofar that we lack the ability to measure such small forces. On the contrary, nanotechnology offers a wide spectrum of techniques that allow us to sense at this scale. However, many natural systems are subject to a noisy environment or need to be surrounded by liquid to maintain their shape and function - which on its own account drastically limits the achievable sensitivity of measurement methods.

Graphene, a single layer of carbon atoms, shows extreme strength and flexibility at the 2D limit of miniaturization. We have rationalized that graphene membranes are a perfect candidate to play the role of flexible support for detection of minute forces in nature, that are often hidden behind the veil of the environmental noise. Graphene owes its suitability to its ultimately thin nature, its low stiffness but simultaneously high tensile strength that prevents it from breaking under high tension. The limits of sensitivity can now be pushed further so that nanoscale forces can be measured in liquid - from pneumatic forces of attoliter volumes of gas, down to the level of single living bacteria.

In this thesis the motion of graphene membranes is studied under the influence of external forces. The motion is detected by a reflectometry setup devised for the study of optomechanical systems immersed in fluid. In Chapter 1 an introduction is given to the topic and the experimental methods are described. In Chapter 2, gases are pumped through a milled nanometer orifices in graphene membranes. The pneumatic interaction and the escape of the gasses through the nanometer scale pores is studied. In Chapter 3, we probe the nanomotion of single bacteria adhered to the surface of a graphene drum. The interplay between the processes occurring at cellular level and the motion of the suspended graphene with bacteria deposited on top is investigated. In Chapter 4, we study the signals obtained when motile bacteria cross a focused laser beam. We also find, that we can enhance the signal by patterning substrates to localise the bacteria close to the laser spot. Finally, in Chapter 5 we give prospects and outlooks, both on application of graphene drum enabled nanomotion sensing for rapid drug susceptibility testing, as well as on further research that might offer new insights into biological processes that can be held accountable for bacteria nanomotion. Furthermore, we discuss developments that would allow for further improvement of the current measurement system that go beyond bacterial sensing.