Tutorial: Contact Angle Analysis (Sliced Method)

This tutorial explains how to compute the contact angle of a droplet using the sliced method in wetting_angle_kit.

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1. Overview

The sliced method divides the droplet into slices (along the z-axis) and fits a geometric model (e.g. spherical) to the liquid–solid interface profile. This is ideal for study the evolution of the angles among a trajectory.

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2. Requirements

Before running the example, ensure you have installed:

pip install wetting_angle_kit ase numpy

Example trajectory:

tests/trajectories/traj_spherical_drop_4k.lammpstrj

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3. Example Code

# Import necessary modules
from wetting_angle_kit.parser import DumpParser, DumpWaterMoleculeFinder
from wetting_angle_kit.contact_angle_method import contact_angle_analyzer

# --- Step 1: Define the trajectory file ---
filename = "../../tests/trajectories/traj_spherical_drop_4k.lammpstrj"

# --- Step 2: Initialize the water molecule finder ---
wat_find = DumpWaterMoleculeFinder(
    filename,
    particle_type_wall={3},  # Wall particle types
    oxygen_type=1,  # Oxygen atom type
    hydrogen_type=2,
)  # Hydrogen atom type

# --- Step 3: Identify oxygen atom indices ---
oxygen_indices = wat_find.get_water_oxygen_ids(frame_index=0)
print("Number of water molecules:", len(oxygen_indices))

# --- Step 4: Initialize the parser ---
parser = DumpParser(filename)

# --- Step 5: Create the contact angle analyzer ---
# Using the 'sliced' method with a spherical model
analyzer = contact_angle_analyzer(
    method="sliced",
    parser=parser,
    output_dir="result_dump_spherical_sliced",
    atom_indices=oxygen_indices,
    droplet_geometry="spherical",  # Geometry fitting model
    delta_gamma=20,  # Smoothing parameter
)

# --- Step 6: Run the analysis ---
results = analyzer.analyze([1])  # Analyze frame 1

# --- Step 7: Display results ---
print("Analysis results:", results)

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4. Expected Output

After running the example, you’ll see something like:

Number of water molecules: 423
Analysis results: {'frame': 1, 'contact_angle': 104.7, 'fit_quality': 0.96}

If plotting is enabled, a visualization of the droplet profile and the fitted spherical interface is generated in result_dump_spherical_sliced/.

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5. Tips

• Use droplet_geometry='spherical' for droplets and droplet_geometry='cylinder_y' for cylindrical droplet on the y axis or 'cylinder_x' for cylinder on the x axis.

• Adjust delta_gamma for smoother or sharper slicing (larger = smoother).

• To analyze multiple frames:

results = analyzer.analyze(range(0, 50, 10))

• Output files include raw interface data and optional plots (if enabled).

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6. Related Files

Example Script: docs/examples/contact_angle_sliced/example_sliced.py

"""
Example: Contact Angle Analysis Using the Sliced Method

This example demonstrates how to perform a contact angle analysis
using the 'sliced' method on a spherical droplet from a LAMMPS dump trajectory.
"""

from wetting_angle_kit.parser import DumpParser, DumpWaterMoleculeFinder
from wetting_angle_kit.contact_angle_method import contact_angle_analyzer

# --- Step 1: Define input trajectory ---
filename = "../../tests/trajectories/traj_spherical_drop_4k.lammpstrj"

# --- Step 2: Identify water molecules ---
wat_find = DumpWaterMoleculeFinder(
    filename, particle_type_wall={3}, oxygen_type=1, hydrogen_type=2  # Wall atom types
)

oxygen_indices = wat_find.get_water_oxygen_ids(frame_index=0)
print(f"Number of water molecules: {len(oxygen_indices)}")

# --- Step 3: Initialize parser ---
parser = DumpParser(filename, particle_type_wall={3})

# --- Step 4: Create analyzer for the sliced method ---
analyzer = contact_angle_analyzer(
    method="sliced",
    parser=parser,
    output_dir="result_dump_spherical_sliced",
    atom_indices=oxygen_indices,
    droplet_geometry="spherical",  # Fitting model
    delta_gamma=20,  # Smoothing parameter
)

# --- Step 5: Run analysis ---
results = analyzer.analyze([1])  # Analyze frame 1

# --- Step 6: Display results ---
print("Analysis results:", results)