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
16
Q
Cohesive and adhesive forces
A
- Cohesive forces (surface tension) try to minimize surface area and form a sphere
- Adhesive forces (btw liquid and surface) pull the liquid down and flatten the drop
17
Q
Hydrophobic vs hydrophilic
A
Hydrophobic - large contact angle, low adhesive (more cohesive forces)
Hydrophilic - low contact angle, high adhesive, surface is easily wetted
18
Q
Bond number, B0
A
- The importance of gravity to surface tension
- Lower B0 -> surface tension dominate, microfluids r –> small B0
- Hydrophilic micro-channel fills easily
- Hydrophobic micro-channel fills difficulty but empty easily
19
Q
Magnetic field
A
=> Moving electrical charges
- When a field is generated in a volume space –> change in energy of that volume
- -> magnetization will min the total energy of the material:
- energy exchange btw spins of närliggande/adjacents atom
- anisotropy energies (directionally energy)
- zeeman energy (potential energy of the magnetized body in an external field
- The effect of magnetic field can be detected by:
acceleration of an electric charge moving in the field
the force of an current-carrying conductor
force and torque on a magnetic dipole
20
Q
Diamagnetic, paramagnetic, ferromagnetic
A
- no unpaired electrons, tiny attenuation of external field
- small #unpaired e-, small intensification
- large # unpaired e- –> large intensification, some can retain magnetic field (permanent magnet
Ferrormagnetic material in a strong magnetic field:
- SATURATION - when all the domain wall are reoriented parallell to the applied field –> net field is generated. When saturation, increasing the applied field has no more effect, the field is “mättnad”
21
Q
soft and hard magnetic material
A
- In soft magnetic material (<1000A/m), the domain walls will again reorient at random orientation when the external field is removed.
- the influence of the field and the body shape must be taken into account
- M for soft material is depending on the internal field
- in hard magnetic material, the domain wall will remain reoriented = permanent magnet
- once magnetized (permanent) M will stay constant and independent of H
22
Q
The Hysteresis Loop (B-H or M-H diagram)
A
H - internal field/magnetizing force (x-axis)
B - Flux density (y-axis) = my_0 *(H+M)
M - Magnetization (vector field)
- Change of H –> determine the hardness or softness of the material: harder magnetic material <=> higher coercivity
- Saturation: M will stay constant, B will increase linearly with H
- Strong permanent magnet: high remanence
- Memory storage: high coercivity (stability) and remanence (SNR)
- Transformers: soft with small hysteresis loop (energy loss in AC)
- Electromagnet: soft (linear behavior), high permeability and saturation (amplification)
23
Q
Anisotropy energies
A
- Try to align the magnetization vector in a given direction
- SHAPE ANISOTROPY: interaction btw the dipoles in the material and tries to align the magnetisation along the geometry of the body
- Higher anisotropy => higher demagnetizing factor, N
- Epsilon case: M will lies between the easy axis (along the body) and the applied field. Higher anisotropy compare to the applied field–> M is closer to easy axis. Higher field strength –> M is closer to the H field.
- if the magnet were free to move, the torque would act to eventually align the easy axis and H.
- if M align to easy axis = Torque, perpendicular to M and H => no driving torque
- If H and M are aligned => the torque is vanished, the force is MAX.
- force will be vanished when: the field is constant along the magnetization direction, M is perpencidular to the spatial derivative of H in that directiion
(- if M is perpendicular to easy axis (swimming direction): driving torque around z-axis. Max torque when H_x is 90° from M => when M is perpendic. with the easy axis and the applied field H, it gives the max driving torque) = permanent magnet
(-depends on its internal field and the torque relationship is nonlinear) = soft magnet
