Forces Flashcards
1
Q
Shrinking length –>
A
- Surface-to-volume ration (S/V)
- relative strength of external forces are not intuitive
2
Q
Example - S/V
A
Example
- Small mammals - heat loss L^2, heat generation (through eating) is L^3
- Capillary tubes: weight scales L^3 and surface tension L^1
3
Q
S/V
A
- Higher for smaller obj. (think about the cubes)
- mass become smaller:
- -> inertial effects become less important ( change velocity quickly, resonant freq. (självsvängning/oscillation) go very high)
- -> gravity = less important
4
Q
Thermal effects
A
- Energy required to heat a volume ~ L^3
- Heat transfer through surface ~ L^2
- Time for thermal eq. in a system ~ L^2 (Biot number describes uniformity of temperature inside body)
5
Q
Thermal microgripper
A
Lower thermal budget:
- Small thermal mass –> consume less power
- Switch on and off much faster (velocity change)
- Biot numb. is smaller –> does not crack that easily
6
Q
Mechanical effects vs. mass sensing resonators
A
- small changes in mass –> detectable changes in resonant freq.
7
Q
Electrostatics (hur elektrisk laddningar påverkar varandra)
A
- Many micro- and nanoactuators are electrostatics.
- Long range interactions, much stronger than most other non-bonded interactions (gravitational)
- Easy to be induced by ionization or polarization
- Decays much slowly than other interactions
Q ~ L^2
Voltage V ~ L^1
Force F ~ L^2
8
Q
Magnetics
A
- Magnetic forces btw current carrying wires: F ~ L^4
- Force of a magnet on a current carrying wire: F~ L^3
- Torque between two magnets ~ L^3
- Force between two magnets ~ L^2
- Force/weight required to lift an obj against gravity ~ L^-1 (smaller mass higher force/weight)
9
Q
Chemistry
A
- Most chemical reactions are surface reactions
- Higher efficiency with larger S/V ratio (smaller mass -> higher efficiency)
10
Q
Power
A
- Onboard power (battery, combustion engine) is bad: power scales ~ L –> cannot provide enough power
- Alternative: light, heat, electric field, magnetic field, chemical reaction
11
Q
Contact Methods 6st
A
- Roughness change - Van der Waals
- Surface tension
- Vacuum - FluidFM (underpressure at the tip to pick up cells)
- Eletrostatic - nanomanipulator
- Conventional pick-up + adherence reduction
- Impulsive
12
Q
Electromagnetic force
A
- Force between charges and/or magnetised matter - (proton/positive charged attracts to electron/negative charged and repulses to same charges)
- Force proportional to the product of the charges q1, q2 (along the lines joining them)
- and inversiely proportional to the square of the distance
13
Q
eletromagnetic forces at atomic scale (7)
A
ionic bond (NaCl) - electron donation
metallic bond (bulk iron)
covalent bond (diamond) - “electron sharing, H_2”
van der waals forces
hydrophobicity
hydrogen bond
solvation forces (interaction btw ionized and uncharged molecules)
14
Q
Van der Waals interaction
A
- is a result of electron charge distribution of the two atoms
- are relatively weak
- interaction btw atoms, molecules and surfaces
Atoms with permanent dipoles:
- dipole-dipole interaction (potential energy ~ r^-3)
- dipole-induced dipole (pot. energy ~ r^-5)
Atoms without permanent dipoles:
- Transient charge distribution induces complementary charge distribution (pot. energy ~ r^-6)
- Repulsion btw two atom when they apporach each other die to overlapping electron clouds (pot. energy ~ r^-12)
15
Q
Surface tension
A
- cause by the cohesive forces within a liquid (attraction btw molecules by various intermolecular forces)
- Inside liquid = equilibrium, at the surface:
1. repulsive forces: thermokinetic energy
2. cohesive forces: wan der waals forces
3. cohesive forces: polar forces - “Cohesion tries to minimize the surface area”
- Surface tension force: parallell to the surface and perpendicular to the contact line
- Low surface tension –> high adhesive force (water) –> capillar effect (h = larger/positive)
- High surface tension –> high cohesive force (Hg) compare to the adhesive forces–> interface pushed down, similar to droplet formation on
surface.
Controlling the surface tension
- electrowetting - change of ST with electronic polarization of the surface
- Termocapillary effect - ST depends on the temperature. Increasing ST towards colder regions
- Hydrophilization induces polarity on the surface - Plasma activation (surface reactions with reactive molecules). Coating with hydrophilic materials
