Engineering and biotribology
L1: Surface characteristics
Lecture 1
14-11-2022
Introduction engineering tribology
Talk about sliding interaction → unlike bacterial adhesions etc.
Tribology → friction, lubrication & wear. Concerned about interfaces.
- Tissue-tissue sliding interaction. E.g. in the lungs, a heart that beats, blinking with eyes or
cartilage in joint.
o Effortless sliding motion is possible due to special structure and surrounding fluid.
For example when this is not the case/does not work →osteoarthritis. With Sjögrens
Syndrome a lot of these fluids are attacked due to immune-mediated attack, which
has physical effects.
- Biomaterial-tissue interaction. E.g. contact lenses. There is a group of people that cannot
handle this. There is a natural lubrication system in the eye that changes when a biomaterial
is placed on it. For some people this change causes irritation.
- Material-tissue interaction. It is not a biomaterial, but things like your phone when you touch
it with your finger. The science of that is called skin tribology.
- Biomaterial-biomaterial. Sliding between two biomaterials. In artificial joints for example.
Metal with metal or metal with polymer or ceramic on ceramic for hip joints. When a
biomaterial slides vs. a biomaterial, you get wear, which is microns getting loose.
Later on in the course we will talk about the biotribology part.
Open book exam + teaching assignment (in group). May bring book, PowerPoint slides & notes. Task
is 25 min lecture. Group 6.
Continued
What are the surface properties that could be affecting sliding interactions between surfaces. There
are two types of surface characteristics:
- Physical
o surface roughness
o stiffness
- Physio-chemical – composition
o Hydrophobicity (water contact angle or surface energy)
o Charge (positively and negatively charged surface; streaming potential)
Surfaces
➔ Differences in surface finishing can change the interaction.
➔ Either physical (roughness, stiffness) or physical-chemical (composition).
Surface charge is measured in streaming potential. Hydrophobicity is measured in water contact
angle and surface energy. Surface energy is like surface tension. It has
the same quantity and measure.
Static charge in dry conditions, like static hair after sliding. What
would happen if a charged surface is put in liquid/aqueous
environment? → there will be water, minerals, charged ions. Minerals
and ions are suspended in the environment, especially a lot of
sodium-chloride ions. If we look at a surface that is negatively charged
(most in nature are negatively charged), then due to coulombic
attraction or repulsion, the + charged ions will be attracted to this
,surface. You are then generating a potential differences (streaming potential). You will get an
electrical double layer (EDL). This the potential that drops (see image). The surface charge is
balanced by the charge in aqueous phase.
The profile that is lower is about freely floating particles. The electrical double layer thickness is 1/k.
When the potential layer is 0, it means that there is no potential, because the concentration of + and
– is the same. At the beginning the concentration of + is more. So what would happen when + and –
ions are added? → The more ions will ‘push on it’, so the electrical double layer will become smaller.
Own explanation: the further away from the negatively charged surface, the potential goes to 0. This
means that there is less difference in charge, so + and – amount is becoming equal. Close to the
negatively charged surface, there is more + ions, so bigger potential. When more – and + ions are
added, the potential becomes 0 less far away from the surface compared to before, because they
‘push on it’. EDL becomes smaller (1/k smaller).
How do we actually measure/quantify the surface? This is the streaming potential. This is one
number. How to measure:
- you bring 2 surfaces of the same charge close to each other. We try to minimize the gap.
- The counter ions will become more and more and the ions, which are the same charge as the
surface, will go out of the space (start moving). You force the liquid to go in one direction.
- There is then a certain charge flowing in one direction, you get a current (Istr), which you can
measure. This is the streaming potential/ Zeta-potential. It is the potential at a certain point
from the surface in the profile in the graph (see slide). This is the boundary of movement vs
no movement (as the negative charged ones go away from the surface).
It is the boundary between the fluid moving and the fluid attached to the surface.
The zeta potentials becomes more important when there are less ions/charges. The other way
around: It disappears when there a lot.
, Hydrophobicity
=surface wetting. Less surface wetting is hydrophobic. It Is the dislike of a
surface to come in contact with water. It is quantified in terms of the
angle the drop makes with the surface (water contact angle (1)). The
more complete definition is with surface energy or surface tension
(mJ/m2)(2). The triple points is the point between the surface, droplet
and air. There is tension between each two of the three. The larger the
theta, the more hydrophobic the surface is. Liquid, solid & interaction
(solid liquid) tension. Formula: , in which
γL = liquid surface tension/energy;
γs = solid surface energy;
γsl = solid-liquid interfacial energy;
θs = solid surface contact ange;
The interfacial energy is based on the solid & liquid. However, you cannot simply add up the energy
of the solid and liquid to get the solid-liquid. There are 3 approaches to get the tension of it.
Different approaches to obtain γsl:
1. Equation of state approach:
2. Geometric mean approach:
a. Surface energy is divided two parts: Van der Waals (apolar; notated with LW) & Acid-
Based (polar, notated with AB). Based on the tension from the solid and the liquid
has on v.d. Waals and Acid-Base, there is then a formula for the solid-liquid
interfacial energy.
i. (a)polarity→ Lipid molecules are apolar (hydrophobic) & more hydrophilic is
polar. A surfactant molecule has both parts for example.
3. LW-AB approach:
There is a last approach broke down the Acid-base part for the polar part. They split it in and
electron acceptor (γ+) and an electron donor (γ-) component.
a. When using LW-AB, you get γs by: and γsl by the one above.
b. Eventually, the get the angle from the following equation: ,
the following is used for the LW-AB approach:
Values that might be needed for the exam:
L1: Surface characteristics
Lecture 1
14-11-2022
Introduction engineering tribology
Talk about sliding interaction → unlike bacterial adhesions etc.
Tribology → friction, lubrication & wear. Concerned about interfaces.
- Tissue-tissue sliding interaction. E.g. in the lungs, a heart that beats, blinking with eyes or
cartilage in joint.
o Effortless sliding motion is possible due to special structure and surrounding fluid.
For example when this is not the case/does not work →osteoarthritis. With Sjögrens
Syndrome a lot of these fluids are attacked due to immune-mediated attack, which
has physical effects.
- Biomaterial-tissue interaction. E.g. contact lenses. There is a group of people that cannot
handle this. There is a natural lubrication system in the eye that changes when a biomaterial
is placed on it. For some people this change causes irritation.
- Material-tissue interaction. It is not a biomaterial, but things like your phone when you touch
it with your finger. The science of that is called skin tribology.
- Biomaterial-biomaterial. Sliding between two biomaterials. In artificial joints for example.
Metal with metal or metal with polymer or ceramic on ceramic for hip joints. When a
biomaterial slides vs. a biomaterial, you get wear, which is microns getting loose.
Later on in the course we will talk about the biotribology part.
Open book exam + teaching assignment (in group). May bring book, PowerPoint slides & notes. Task
is 25 min lecture. Group 6.
Continued
What are the surface properties that could be affecting sliding interactions between surfaces. There
are two types of surface characteristics:
- Physical
o surface roughness
o stiffness
- Physio-chemical – composition
o Hydrophobicity (water contact angle or surface energy)
o Charge (positively and negatively charged surface; streaming potential)
Surfaces
➔ Differences in surface finishing can change the interaction.
➔ Either physical (roughness, stiffness) or physical-chemical (composition).
Surface charge is measured in streaming potential. Hydrophobicity is measured in water contact
angle and surface energy. Surface energy is like surface tension. It has
the same quantity and measure.
Static charge in dry conditions, like static hair after sliding. What
would happen if a charged surface is put in liquid/aqueous
environment? → there will be water, minerals, charged ions. Minerals
and ions are suspended in the environment, especially a lot of
sodium-chloride ions. If we look at a surface that is negatively charged
(most in nature are negatively charged), then due to coulombic
attraction or repulsion, the + charged ions will be attracted to this
,surface. You are then generating a potential differences (streaming potential). You will get an
electrical double layer (EDL). This the potential that drops (see image). The surface charge is
balanced by the charge in aqueous phase.
The profile that is lower is about freely floating particles. The electrical double layer thickness is 1/k.
When the potential layer is 0, it means that there is no potential, because the concentration of + and
– is the same. At the beginning the concentration of + is more. So what would happen when + and –
ions are added? → The more ions will ‘push on it’, so the electrical double layer will become smaller.
Own explanation: the further away from the negatively charged surface, the potential goes to 0. This
means that there is less difference in charge, so + and – amount is becoming equal. Close to the
negatively charged surface, there is more + ions, so bigger potential. When more – and + ions are
added, the potential becomes 0 less far away from the surface compared to before, because they
‘push on it’. EDL becomes smaller (1/k smaller).
How do we actually measure/quantify the surface? This is the streaming potential. This is one
number. How to measure:
- you bring 2 surfaces of the same charge close to each other. We try to minimize the gap.
- The counter ions will become more and more and the ions, which are the same charge as the
surface, will go out of the space (start moving). You force the liquid to go in one direction.
- There is then a certain charge flowing in one direction, you get a current (Istr), which you can
measure. This is the streaming potential/ Zeta-potential. It is the potential at a certain point
from the surface in the profile in the graph (see slide). This is the boundary of movement vs
no movement (as the negative charged ones go away from the surface).
It is the boundary between the fluid moving and the fluid attached to the surface.
The zeta potentials becomes more important when there are less ions/charges. The other way
around: It disappears when there a lot.
, Hydrophobicity
=surface wetting. Less surface wetting is hydrophobic. It Is the dislike of a
surface to come in contact with water. It is quantified in terms of the
angle the drop makes with the surface (water contact angle (1)). The
more complete definition is with surface energy or surface tension
(mJ/m2)(2). The triple points is the point between the surface, droplet
and air. There is tension between each two of the three. The larger the
theta, the more hydrophobic the surface is. Liquid, solid & interaction
(solid liquid) tension. Formula: , in which
γL = liquid surface tension/energy;
γs = solid surface energy;
γsl = solid-liquid interfacial energy;
θs = solid surface contact ange;
The interfacial energy is based on the solid & liquid. However, you cannot simply add up the energy
of the solid and liquid to get the solid-liquid. There are 3 approaches to get the tension of it.
Different approaches to obtain γsl:
1. Equation of state approach:
2. Geometric mean approach:
a. Surface energy is divided two parts: Van der Waals (apolar; notated with LW) & Acid-
Based (polar, notated with AB). Based on the tension from the solid and the liquid
has on v.d. Waals and Acid-Base, there is then a formula for the solid-liquid
interfacial energy.
i. (a)polarity→ Lipid molecules are apolar (hydrophobic) & more hydrophilic is
polar. A surfactant molecule has both parts for example.
3. LW-AB approach:
There is a last approach broke down the Acid-base part for the polar part. They split it in and
electron acceptor (γ+) and an electron donor (γ-) component.
a. When using LW-AB, you get γs by: and γsl by the one above.
b. Eventually, the get the angle from the following equation: ,
the following is used for the LW-AB approach:
Values that might be needed for the exam:
