Piezoelectric Top Mount

The Piezoelectric attachment is part of our SFA2000 Basic System. This attachment supports normal force studies. Unit provides the most-fine surface separation control at 1nm/V over 1um range (1000 V maximum). Typical distance resolution of 0.1 nm (1 Å). System speed range: <0.1 Å/sec to >1 mm/sec (lower limit depends on drift, upper limit depends on inter-surface material viscosity).

Schematic: Cross section view of the Piezoelectric attachment mounted on the Main Chamber with the Main Translation Stage Bottom Mount hosting the lower surface.

Friction Device Top Mount

The Friction Device Attachment is a ‘complete unit’ – both producing and measuring shear motion – and is an improved version of the unit originally described by Israelachvili et al. [Science 240 (1988) 189]. Sliding motion of the upper surface is produced via a digital encoder-controlled motor-driven micrometer, and the shear/friction force acting on that surface is measured with a Wheatstone Bridge connected to resistance or semiconductor strain gauges (red in schematic) attached to the friction-force-measuring double-cantilever springs supporting the upper surface. Assembled Friction Device includes: machined and finished parts, micrometer and reversible DC motor with digital encoder and display counter for lateral displacement control and measurement, force-measuring strip springs with strain gauges mounted and wired to an external Lemo connector, silica disks, clamping rings for attaching to the Main Chamber. Wheatstone Bridge with amplifier and a chart recorder or digital signal capture are required ancillary equipment.

Schematic: Cross section view of the Friction Device mounted on the Main Chamber, the Bottom Mount hosting the lower surface is not shown.The surfaces can be sheared past each other by using a motor-driven micrometer to move the upper surface in the lateral direction. The motorized Friction Device has a reversible dc motor, M, that can be switched on or off or driven in reverse at different constant or variable speeds using a dc power supply or function generator. When using a function generator, a square wave voltage function must be used to shear the upper surfaces back and forth at a constant +/- velocity.

3D Sensing / 1D Actuator Top Mount

The 3D Sensor/1D Actuator Attachment provides linear motion for the upper surface over a 1um range in the z-direction using a piezoelectric tube. The 3D sensor can measure the force in any spatial direction. This 3D force sensor has been used successfully in various studies of thin film adhesion, friction and lubrication. [Kristiansen et al., Adv. Mater. (2012), 24, 5236-5241], [Lee et al., PNAS (2013), E567-E574], [Charrault et al., Tribol. Let. (2013) 50, 421-430].

​Schematic: Cross section view of the 3D Sensor/1D Actuator mounted on the Main Chamber, the Bottom Mount hosting the lower surface is not shown. Translation in the vertical, Z-direction, is generated by the piezoelectric tube (red-dotted) supporting the upper surface. The upper surface is suspended from four vertical wire springs (solid red) and four horizontal cantilever springs, each mounted in a square/cross configuration, each with one or two strain gauges on them, allowing for Z deflections (loads) and hence the forces experienced by the upper surface to be measured in the X, Y (vertical wire springs) and Z directions.

Main Translation Stage Bottom Mount

The Main Translation Stage Attachment is part of our SFA2000 Basic System. This attachment supports normal force studies. The range of force-measuring spring stiffness attainable spans 4 decades, from 30 N/m to 5×10^5 N/m. Maximum compressive pressure of 0.5 MPa (5,000 atm) using curved surfaces of radii 0.5 – 1 mm.

Schematic: Top view of the main translation stage and lower disk mount. Removal of the motor, fine micrometer, coil spring and anti-backlash spring and screws I, II and III (which bolt the main stage to the upper chamber wall) allows for the main stage and attachment base (green) to be rotated anti clockwise about screw IV for easy access to the lower surface.

Bimorph Slider Bottom Mount

The Bimorph Slider Attachment produces only lateral (or shear) motion of the lower surface via two parallel sectored bimorphs in a ‘double-cantilever’ geometry. Sinusoidal, saw-tooth, step-function and other types of displacement-time functions can be generated over a frequency range from micro-Hertz to kilo-Hertz, corresponding to sliding speeds from ~10 cm/s to ~10-8 cm/s. This dynamic range is much greater than can be attained with the motor-driven Friction Device. [Luengo et al., Macromolecules 30 (1997) 2482–2494]. The assembled unit includes bimorphs and wiring to an external Lemo connector, a fixed-stiffness normal force-measuring double-cantilever spring with disk mount and two silica disks, and a means for mounting the unit to the SFA Basic Unit.

Schematic: Cross section view of the Bimorph Slider Attachment mounted in the Main Chamber, also shown is the Friction Device mounted on the chamber. Bimorph Sliders can be used simultaneously with the Friction Device, which must be used if shear/friction forces are also to be measured. Function generator for driving the bimorphs not provided.

Variable Spring Bottom Mount

The Variable Spring Attachment utilizes an adjustable sliding clamp mechanism (yellow) on the force-measuring springs to allow in situ adjustment of the force-measuring double-cantilever spring. Decreasing the free-end length of the spring by a factor of 10 will lead to an 1000-fold increase in the stiffness allowing the experimenter to drastically alter the spring stiffness by simply adjusting the active spring length. This attachment addresses situations where the system being investigated involves a combination of strong and weak forces, or if the system is completely unknown and selecting a priori a fixed force-measurement spring is difficult.

Schematic: Cross section view of the Variable Spring Attachment mounted in the Main Chamber, also shown is the Friction Device mounted on the chamber. A sealed side-port feedthrough allows for adjustment of the force-measuring springs.

High-Speed / Large Displacement Rotating Disk

The High-Speed / Large Displacement Rotating Disk Attachment provides a much longer range and higher sliding speeds than our other friction devices. This device is inherently a sphere/flat geometry (spherical upper surface and flat lower surface) where the lower flat surface is a transparent or opaque flat disk connected to a compact DC motor (not shown) allowing for sliding velocities up to around 5 m/s (i.e. about 6 orders of magnitude faster than current piezoelectric bimorph sliders). Different DC motors offering a range of rates and stalling torques with the same form-factor for fitting within the SFA Main Chamber are available. This attachment relies on a unique plane adjustment system for the disk surface. When using an opaque surface as the bottom surface for this attachment (or any other attachment), a modification of the optical arrangement is required for observing the fringes of equal chromatic order (FECO) in reflected light rather than in transmitted light.

Schematic: To adjust the level, the disk is mounted on a copper–beryllium (Cu–Be) ring or wheel that is held to the central shaft by stiff arms. Below these are wire springs (red), three of medium stiffness and three of low stiffness. By adjusting the six screws, one can adjust the angle of the disk, first coarsely then finely, to give a surface that rotates with little wobble per rotation.

Bimorph Vibrator Bottom Mount

The Bimorph Vibrator Attachment is an attachment for vibrating one surface (vertically) while measuring – for example, with a lock-in amplifier – the amplitude and phase of the vibrations induced in the other surface. Useful for measuring rheological and visco-elastic properties of fluids and thin fluid films near or between two surfaces [Israelachvili, Kott and Fetters, J. Polymer Sci., Part B: Polymer Physics 27 (1989) 489-502]. Assembled unit includes all bimorph wiring to external Lemo connector, disk mount, and silica disks. Function generator, frequency counter and lock-in amplifier not provided.

​Schematic: Cross section view of the Bimorph Vibrator Attachment mounted in the Main Chamber. An AC drive voltage on the upper surface (Piezoelectric top mount for example) and monitoring the voltage across the bimorph strip (red) provides a measure of the induced motion of the lower surface.

For specialized attachments, click here.