Soft contact tribology is at the heart of our feelings perceived as moistness, smoothness and comfort. Although there is no standard definition, the study of friction behavior of low modulus materials could qualify as soft contact tribology. Lately, several studies have highlighted the benefits of soft contact tribology in improving our perception of feelings, through engineering the materials that we touch.
Friction (or shear stress) connects the mechanical action like touch with the psychological reaction such as smoothness feeling. A representative friction curve for the soft contacts is given in Figure 1.
Few important features in this curve are static friction and kinetic friction. The ratio of static to kinetic friction is used to define comfort feeling. Friction is influenced by the adhesion and deformation at the contact, as shown in this representative curve (Figure 2).
Such friction is investigated in the lab using a tribometer – a friction measuring device.
Figure 1. Typical friction curve for soft contacts.
Figure 2. Relationship between friction coefficient and normal load in soft contact applications.
What is a good tribometer for soft contact tribology?
There are several characteristics like resolution, noise level, accuracy, etc. However, an important difference is with the “purity” of friction data acquired in a tribometer: it is only possible if the friction force is measured independently of normal force. There should be minimum or no cross talk between them. As an example, let us look at our patented friction measuring system in MicroForce (Figure 3).
Ducom MicroForce applies a normal force ranging from 10 mN to 10 N through a stepper motor and a piezoelectric actuator, for coarse and fine movements, respectively.
The applied force is measured by means of two horizontal spring sheets, whose deformation, measured by a capacitive sensor, is directly proportional to the normal load. Similarly, the tangential or friction force is measured by two vertical spring sheets. It measures friction coefficient as low as 0.001 at a rate of 2000 data points per second. This patented system acquires normal force and friction force independently from one another.
Figure 3. Ducom MicroForce (left) and schematics of the patented sensing system (right).
Examples of soft contact tribology applications using the Ducom MicroForce
Two Foley catheters (Figure 4) were tested by reciprocating a hydrophobic silicon rubber (PDMS) while being submerged in water. Catheter B had a hydrophilic coating.
Catheter B resulted in a much lower friction than Catheter A and, consequentially, a lower energy needed to insert the catheter, thus making Catheter B a more comfortable solution for a patient.
Figure 4. Views of the test setup (left) and friction loops representation of Catheter A and Catheter B (right).
Measuring touch sensations by consumer products
Consumer products like cosmetics are described by primarily subjective parameters (moistness, greasiness, thickness, slipperiness, etc.). Ducom MicroForce could distinguish different consumer products based on their tribological properties, as shown in Figure 5.
The two commercial moisturizing creams differed in their consistence, with Cream B much thicker than Cream A. Such difference was precisely measured by a rubbing test performed by three different subjects on a clean glass with the interposition of a thin layer of cream.
Figure 5. Schematics of a finger test (left) and average friction coefficient obtained by the three subjects (P1, P2, P3) on bare glass and cream
Shaver blade coating evaluation
The resolution of the friction measurement was crucial to quantify the stress a coating could endure before failure in a peeling test. As shown in Figure 6, a staircase load was applied to identify the failure of different coatings under a reciprocating movement with extreme precision.
Figure 6. Representative graph of normal load (top), friction coefficient of a failed coating (middle) and friction coefficient of a successful coating (bottom).
Friction is the main factor affecting the tribological behavior of soft contacts. A useful measurement of friction must be extremely precise and independent from noise and cross-talk with, for instance, normal force measurement. Ducom MicroForce provides the fine measurements needed in such applications and the versatility necessary to measure friction in different test setups relevant for soft contact tribology.