Rheoprinter

"The Rheoprinter": Online learning of printability conditions

2. Proposed System: "The Rheoprinter"

1. Background

1.1 Motivation & Initial Project Idea: "A Desktop Rheometer?"

$$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$

/{x^i}_2/ Material Measurements for the Masses
Understanding the Rheology of Complex Fluids

Transforming a peptide gel (99% water) into a yield-stress material for 3D pritning.

"YIELD STRESS": An ever increasing viscosity as the shear rate approaches zero, i.e. does not flow/ solid-like when stationary
"ZERO-SHEAR VISCOSITY": The viscosity plateau's as teh shear rate approaches zero, i.e. flows/ liquid-like when statioanry

My optimistic end goal for this project is the making of a suite of rheometers that will allows us to measure rheological material functions of different types of complex fluids (including the ones with temperature-dependent rheological properties) at the whole range of shear rates as it is depicted in the following figure:

So, the goal would be that if someone gives us a material or we have created our own material, we would be able to poulate the pomymer viscosity vs shear rate graph with points acrosss the whole range of shear rates and for different temperature conditions, if it has temperature-dependent rheological porperties.

What about a **Confocal Parallel Plate or Extrusion Microrheometer** for studying behavior of Complex Fluids?
Here is a nice paper: Lin & Cohen 2014 - Confocal Rheoscope
and
here is a nice video.

I'm thinking about starting with a rotational rheometer

Components:

Linear Actuator

Z-axis

Rotational Stage

θ-axis

Motor Controller x2
Dynamic Torque Measurement Device

Material Measurements for the Masses
Understanding the Rheology of Complex Fluids

1.2 The Science of Rheology

The measurement of rheological properties and the evaluation of fluid models require specific devices that can be summarized as *rheometers*. Rheology is concerned with the behavior of fluids undergoing deformation. This deformation can be shear, elongation, or a combination of deformations such as those occuring in the complex flow field within a mixer. An important aspect of rheology's scope is to find the relation between deformation and stresses for various well defined conditions, such as transient shear flows, step strain, creep and oscillatory shear flow . These relations also called *material functions* are determined using different types of rheometric techniques.

Figure 1 below shows the rate of deformation achievable with the different measurement techniques. In addition, it explains how the rate of deformation corresponds to the time scale of the molecular movement . The diagram also relates the phenomena or properties under investigation and common polymer processes to the different test methods. slow deformations (on the left) only affect local molecular movement such as rotations , while fast deformations disentangle the molecular chains and allow for changes of location of whole molecular segments. The different time scales are also related to mechanical and failure behavior such as creep and impact.

Different manufacturing processes expose the material to varying shear rates. While thermoforming and extrusion subject the material to lower rates of deformation, the injection molding process exposes the polymer melt to rates of deformation as high as 10^5 s^-1 .

Unsteady Shear Flow Behavior

Viscosity is not only dependent on shear rate it is also time dependent
Think of paint: thick in the can when left i the shed for months, but thins when stirred
Think of paint: thick in the can when left i the shed for months, but thins when stirred
However, it is thixotropic as it does not rebuild straight away after stopping the stirring

Two samples...one very thixotropic, one not so thixotropic. Bad paint: leaves brush mark because it rebuilds too thick too quickly. Good paint: leaves smooth finish because it rebuilds quite slowly giving enough tiome to allopw ridges to smooth out.

1.3 Rheometers - Operating Principles

Commercial Rheometers

Rotational rheometer designs at TA Instrumnets.

Relating Instrument Specifications to Material Properties

The measured qunatity (angular deformation and torque) are transferred into a material quantity (stress, strain, viscosity, etc.)

INstrument specific measured quantities are used to calculated material-specific parameters.

Geometry specific constants relate the measured instrument data with the desired material parameter.

Fluid Dynamics of Rotational Rheometry

Working Equations for rotational rheometers with ***parallel plate geometry*** .

Thus, by varying the shear rate and measuring teh change in torque, the viscosity may be determined explicitly.

Notes on Experimental Procedures:

The most important thing is the fundamental resolution of the instrument itself since this will dictate our primary measurements. Stress and strain are not measured directly. Important assumptions are required ot convert primary measurements to material functions. You can only measure certain dynamic ranges of torque, displacement and frequencies. You have resolution limits in either of these quantities you can propagate through to the resolution o fthe viscosity that you can measure.