Nanomechanical sensing in liquid

This thesis describes advances in the field of nanomechanical sensors operating in liquid. Firstly, a
novel method for measuring nanoscale displacements is presented. Secondly, microscale Chladnifigures
are demonstrated on oscillating cantilevers by means of boundary streaming in the aqueous
environment. Thirdly, the physics of boundary streaming is clarified for the first time. The three
topics are summarized below.
A novel displacement sensor based on a squeezable molecular multilayer
A novel displacement sensor for nano- and micro-electromechanical devices (NEMS and MEMS)
is introduced. The technique is based on a squeezable molecular multilayer, combined with electron
tunneling or with capacitive readout. The main advantage is the predefined alignment of
the electrodes, allowing miniaturization of traditional tunneling and capacitive sensors. Furthermore,
the device can be operated in aqueous solutions. The multilayers consisted of stacked selfassembled
monolayers (SAMs) of mercaptohexadecanoic acid. Capacitive measurements revealed
the dielectric constant of the multilayers, which was ²r = 1:5. Squeezing of a bilayer lead to an
exponential change in a tunnel current, resulting in nanometer displacement sensitivity.
Nanomechanical resonators generating Chladni figures and boundary
streaming
Chladni figures based on nanomechanics in the microfluidic environment are presented. In contrast
to the macroscopic observations in the gaseous environment, nanoparticles were found to move to
the nodes, whereas micron-sized particles moved to the anti-nodes of the vibrating interface. This
opens the door to size-based sorting of particles in microfluidic systems, and to highly parallel and
controlled assembly of biosensors and nanoelectronic circuits.
The physics of boundary streaming
The physics of boundary streaming is revealed for the first time. This vortex flow phenomenon occurs
near all oscillating surfaces in fluid media, therefore affecting operation of cantilever sensors
and other nanomechanical devices with oscillating components. Here, a solution to the Navier-
Stokes equation is obtained by using a series of physical analogies, giving full insight into the
physics of boundary streaming.